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What methods of protection exist against spam. Spam protection. Advantages of the proposed solution

What methods are there to combat spam?

There are two main methods of protecting a mail server from spam: protection against spam at the receiving stage mail server and “separation of spam” from the rest of the mail after it is received by the mail server.

Among the first method, the most popular methods are the use of DNS Black List (DNSBL), Greylisting and various delays when sending mail; use of various technical means, such as checking the existence of a user on the sending side (callback), checking the “correctness” of the sending server using such methods as the presence of a record in the reverse DNS zone, the legality of the name when setting up an SMTP session (helo), checking the SPF record (for this to work in DNS records about the host, the corresponding entry about the legitimate sender servers is used).

Among the methods for analyzing the content of a letter, the most popular methods are checking using various algorithms, such as searching for special advertising keywords or based on Bayes’ theorem. The algorithm based on Bayes' theorem contains elements of probability theory, is initially trained by the user on messages that, in his opinion, are spam, and subsequently separates messages that contain spam based on characteristic features.

So, let's look at these filtering methods in more detail. Email.

Black lists or DNSBL (DNS Black Lists)

Blacklists include addresses from which spam is sent. Lists such as “open relays” and “open proxies” are widely used, and various lists dynamic addresses that are allocated by providers for end users. Due to their ease of implementation, the use of these blacklists is done through the DNS service.

Gray lists or greylisting

The operating principle of greylisting is based on spamming tactics. As a rule, spam is sent in very a short time in large quantities from any server. The work of a gray list is to deliberately delay the receipt of letters for some time. In this case, the address and time of forwarding are entered into the gray list database. If the remote computer is a real mail server, then it must store the letter in a queue and resend it within five days. Spambots, as a rule, do not save letters in the queue, so after a short time they stop trying to forward the letter. It has been experimentally established that on average the time it takes to send spam is a little more than an hour. When resending a letter from the same address, if the required amount of time has passed since the first attempt, the letter is accepted and the address is added to the local white list for a sufficiently long period.

Performance Analysis

The first two methods allow you to filter out about 90% of spam at the stage of delivery to the mailbox. Already delivered mail can be marked by means of analyzing the contents of the letter, for example, using the SpamAssassin program. This product allows, based on special algorithms, to add corresponding lines to the headers of letters, and the user, based on mail filters in the mail client, can filter mail into the necessary folders of the mail program.

Conclusion

Of course, there are other methods of protecting against spam, the most effective, unfortunately, on this moment are preventive measures, such as not leaving your real email mailbox on websites, forums and message boards, using temporary addresses for such needs, which can later be deleted, if it is necessary to publish the mailbox on the site, use text instead graphic image and similar measures.

You can connect and configure GreyListing through the ISPmanager panel in the "Features" section

You can find out more about setting up anti-spam methods through the control panel here DNSBL and here Greylisting.

This is a new Kaspersky Lab product designed for comprehensive protection home computer. This program provides simultaneous reliable protection from viruses, hackers and spam. The Kaspersky Anti-Spam module is one of the elements of this home computer protection system. First of all, it should be noted that Kaspersky Anti-Spam is not an independent product and does not work separately from Kaspersky Personal Security Suite. To some extent, this can be called a disadvantage, since users cannot use Kaspersky Anti-Spam separately, but comprehensive protection also has its undoubted advantages.

Anti-virus protection and firewall have been discussed more than once on the pages of our publication. Therefore, in this article we will look exclusively at the operation of the antispam module.

The basis of Kaspersky Anti-Spam is the intelligent SpamTest technology, which provides: fuzzy (that is, triggered even if there is an incomplete match) comparison of the letter being checked with samples - letters previously identified as spam; identification of phrases characteristic of spam in the text of the letter; detection of images previously used in spam emails. In addition to the criteria listed above, formal parameters are also used to identify spam, including:

"black" and "white" lists that the user can maintain;
various features of mail message headers characteristic of spam, for example, signs of falsification of the sender's address;
techniques used by spammers to deceive mail filters - random sequences, replacing and doubling letters, white on white text and others;
checking not only the text of the letter itself, but also attached files in plain text, HTML, MS Word, RTF and other formats.

Installation of the antispam module

The module is installed during installation of Kaspersky Personal Security Suite. When choosing installation options, a user who uses email clients other than Microsoft email programs may not install the module for Microsoft Outlook.

It should be noted that Kaspersky Anti-Spam scans any correspondence received by email SMTP protocol. Thanks to this, it can filter out spam in any email program, but more on that below.

Integration into Microsoft Outlook Express

The program does not have its own interface as such. In Microsoft Outlook Express, the Kaspersky Anti-Spam module is integrated as a menu and as an additional panel.

One may note some inconvenience when using this panel, although it has nothing to do with the antispam module itself. Due to the principles of operation of the mechanism Microsoft programs Outlook Express Kaspersky Anti-Spam panel cannot be docked in a convenient place for the user. Each time you start the program, the panel will appear third. You will have to constantly move it to a convenient place or come to terms with this state of affairs.

Program operation

When receiving mail, Kaspersky Anti-Spam analyzes incoming correspondence. If spam is detected, the letter is marked with a special label [!! SPAM] in the Subject field and placed in the Deleted Items folder. Messages recognized as non-spam are not marked with anything and are processed by the mail program in accordance with established rules. If the program is not sure that the letter is spam, then the [?? Probable Spam] and the letter is placed in the Inbox for the user to make a final decision. In addition, the program uses two more types of labels: - for letters with obscene content and - for automatically generated letters, for example letters from email robots.

Thanks to such tags you can organize Kaspersky work Anti-Spam with any other email program. It is enough to create rules in your email client to sort emails by these tags. In Microsoft Outlook itself, such folders are created with one click of a button in the antispam module settings window.

Training program

The program can be trained in two ways: by classifying messages received by the user as spam - not spam, and by downloading updates from the Laboratory server. The first method allows you to train the program for the user’s personal email, the second allows you to quickly respond to massive spam events on the Internet.

When you launch it for the first time, Kaspersky Anti-Spam will extract from address book Microsoft Outlook all addresses to add them to the "Friends List". All letters from these recipients will be perceived by the antispam module as not spam and will be passed through without checking. Subsequently, the user can edit this list by adding or removing recipients to it. In addition to the "Friends List" there is also a "Enemies List". Any correspondence received from recipients on the Enemy List will be clearly classified as spam.

Adding recipients to your friends or enemies lists is done by simply clicking a special button on the Kaspersky Anti-Spam panel. Training is also carried out there. If you miss a spam email, you just need to click the “This is spam” button. A window will appear in which the user must tell the program what to do with this message.

The “Send as an example of spam” command generates a letter to Kaspersky Lab with a message about spam for further training. This command can be ignored. You can neglect adding the author to the enemies, but you should definitely add the letter to the spam samples. This is how the program is trained for personal correspondence.

Since Kaspersky Anti-Spam does not integrate into other email clients, its training in these programs is only possible through updates received from the Laboratory server. Unfortunately, this training option does not make it possible to train the program for the specifics of personal mail.

Settings

In the program settings you can: specify the location of the module databases, if the user wants them to be stored in a non-standard location; disable or enable filtering; set update parameters and view statistics.

The Kaspersky Anti-Spam module provides sufficient full protection user email from spam. Like any other program, it requires training. And while this learning is happening, erroneous recognitions are possible correct letters like spam and vice versa. A relative disadvantage is that the module does not allow you to delete messages on the server that are obvious spam. The user still has to spend his traffic on these unnecessary letters. On the other hand, with this approach to spam filtering, not a single valuable message will be lost. In all other respects, Kaspersky Anti-Spam deserves the most serious attention, especially considering the integration of the module with other programs that ensure the security of the user’s computer.

Hello, dear readers of the blog site. You've probably already encountered spam, even if you didn't know exactly what this outrage is called. And for this it is not even necessary. Spam also pours into your regular mailbox with enviable regularity - these are the same countless advertising booklets that are slipped to you in the hope that you will order or buy something.

So, spam is just that, an annoying one, creeping in from all sides advertising(but not only advertising - there are worse things). You didn’t order it, you don’t need it, but he rushes and rushes from every corner. The information you need can easily get lost in this heap and you have to spend a lot of time sifting through it.

In our computer age, the main source of spam. And besides advertising spam mailings can also be dangerous both for your wallet (phishing, social engineering, scams) and for your computer (viruses, worms, Trojans).

What does the word Spam mean, what does it look like and how can you fight it, you will learn by looking at this short article. I hope it will be interesting (well, certainly useful - I promise you that).

Spam - what is it?

What does the word SPAM mean?

Self word Spam comes from the name of canned meat, which was vigorously advertised after the end of the Second World War (obviously, it was necessary to urgently sell out the soldiers' meat rations).

The advertising was so aggressive, comprehensive and omnipresent that this word (and the associated “sediment”) was remembered, but only in relation to the one that appeared with it. intrusive advertising in conferences (then still on Fidonet, if anyone remembers).

The word stuck, especially since intrusive advertising did not become less, but rather the opposite. When email gained popularity, unauthorized mass advertising and malicious mailings became commonplace. Such mailings were profitable for spammers, because the necessary information was communicated to a large number of people without any special costs.

But It's not limited to email. They spam in private messages on social networks, on forums, in instant messengers, on message boards, in comments to blogs, open for editing and adding text by anyone. They will also spam your phone, for example by calling you advertising or sending advertising SMS messages.

Where can you find him on the Internet?

Email- it's just a Klondike for spammers. With the help of mass mailings you can sell anything, you can deceive and rob, you can infect computers and send worms. Databases for mass mailings are collected independently (with the help of programs), or purchased from those who do this professionally.
Forums, comments blogs, wiki sites and message boards - here, essentially, everyone is allowed to leave messages and spammers find it difficult to resist spamming. This is not always advertising - often in this way, webmasters try to get a free link to their site so that it ranks higher in the Yandex or Google search results for various queries. This brings them traffic and money.
Social media and dating sites - spam is very common among incoming personal messages. It is also available in the comments to messages.
Messengers (like ) are also susceptible to this scourge.
SMS-messages from unknown people of an advertising nature. Probably everyone is familiar with it.
Search spam- a rather specific thing, but familiar to everyone. Have you ever had a situation where you enter a query into Yandex (Google), and the responses are entirely sites with some kind of bullshit that have nothing to do with the matter. These are so-called doorways (sites with automatically generated useless texts). They spam search engine results, but they make money from visitors visiting them (in different ways).

Those. all letters in your mailbox (or forum messages, blog comments, personal messages) of an advertising or other nature from a sender unknown to you - this is spam. True, the newsletters you have subscribed to can also be intrusive, but you can still unsubscribe from them (refuse to receive them further).

Spam itself is annoying and annoying, because it litters where it appears. Most often, this is your inbox, and it can be quite difficult to separate the wheat from the chaff when there are too many unwanted messages. But spam can also pose a real danger to both you and your computer. Let's figure this out.

Understanding the types of spam (harmless and dangerous)

From now on I will mainly talk about spam messages arriving by email, because it is this channel that is considered especially susceptible to this phenomenon. Everything else is not as neglected and is not so actively used by spammers, and this does not apply to all network users. But the problem of protecting them from unwanted messages is acute.

Types of harmless spam

Advertising of legal goods and services- in this case, the business owner simply chose spam mailings as one of the channels for attracting clients and customers, because it is not expensive, fast and brings results. Naturally, he doesn’t think much about the moral (or rather immoral) side of this matter.
Prohibited advertising— there are goods and services that are prohibited by law from advertising, and for them spam mailings can be the main channel for attracting customers. By the way, spammers also advertise their services through mass mailings, since this activity is prohibited by law. This also includes advertising that discredits competitors, since this is also prohibited by law (praise yourself, but don’t scold others).
Impact on public opinion- very often spam becomes a good choice for those who are trying to manipulate public opinion in the desired direction. It could be politics, but it could also be. Letters in general can be sent supposedly from some person in order to compromise him, change his opinion about him, or use his authority for profit. But this does not pose a danger to you personally.
Letters that ask to be forwarded to others- these can be varieties of so-called “letters of happiness” (send it to 10 friends and you will be happy), or asking to send information to friends for some other reason. Often such letters are used by spammers to collect or replenish an existing email database for subsequent mass mailings.

High-risk spam - what it could be

While regular (harmless) spam may only have a slight impact on your peace of mind and mental health, the examples below can be quite dangerous for your wallet or computer. And this is no joke.

I myself (the one who teaches and trains everyone) a couple of years ago “bought” a similar type of letter (phishing) and parted with several thousand rubles (read “”). I just got wrapped up and did everything “automatically” just to get behind, well, when I realized it, it was too late.

- Very effective method deceiving not only gullible people, but also simply busy or inattentive people (there is a hook for everyone). They send you a letter supposedly from your bank or service electronic money or from somewhere else. In this letter, you will definitely be taken aback by something (unsettled) and asked to log in to the site to solve the problem. You log in, but the site will be fake (although it looks exactly like the real one) and the data you provided will be immediately used to steal all your money.
Nigerian letters- you find out that you can get a large amount of money (various pretexts - from an unexpected inheritance to helping a prince in exile). You don’t believe it for a long time, but they convince you. When you believe it, you will be asked to transfer some money for “related expenses.” You translate and no one will bother you anymore.
Viruses, worms, Trojans— the letter may contain an attached file with a malicious program (or a link leading to a site with a virus). She can immediately cause problems with the operation of the computer, or she will sit quietly in a corner and carefully write down all the passwords, logins and other useful household things you enter. Worms, among other things, can also send themselves to your friends using the email addresses found in your contacts (as they will call you later...).

Spam protection

Where do spammers get databases with email addresses?

Spammers (those who send mass mailings) collect email addresses from all available sources. These could be forums, guest books, chats, social media and other sites where postal addresses may be publicly available.
Hackers manage to get to some databases of addresses stored on websites.
Collecting Emails is carried out in most cases programmatically (with the help of search bots - harvesters) and this does not require much effort (only time, and even then not very much). Moreover, similar spam email databases You don’t have to collect them, but buy them from those who specialize in this matter ().
There are billions of postal addresses registered all over the world, and therefore you can simply try to generate Emails using special programs using appropriate dictionaries. With a high degree of probability, many of them will actually exist. Read below about how spammers check the reality of addresses.
There are special worms (viruses) that can send themselves to a database of addresses found on the victim’s computer. The database collected in this way will already be cleared of non-working mailboxes.

How do spammers clean databases of inactive email addresses?

Those who collect a database of addresses essentially don’t care who owns this or that address - they send letters to everyone, because there will still be someone who will respond to them (as they say - they hit the squares).

But still, in order to optimize costs and increase the return on mailings, it is beneficial for them to clear the databases of non-existent addresses. How do they do it? Let's get a look.

The simplest thing is to place a picture in the letter (maybe not even visible - one pixel in size), which will be loaded from the site owned by the spammer when the user opens the letter. If the picture is loaded, it means the letter was opened and the email is valid.
Many email clients (programs for working with email) automatically send a message that the letter has been read, which again plays into the hands of spammers.
The letter may contain a link urging you to go somewhere, promising mountains of gold. We've moved on - consider that your Email will now be marked as valid. The most unpleasant thing is that such a link can be disguised as an unsubscribe button, which will actually lead to the opposite effect.

How to reduce the likelihood of your email getting into the spam database?

In general, as soon as your mailbox is “verified,” spammers won’t just leave you. It is important to understand that no one is immune from spam. But it is possible significantly reduce the likelihood of getting into such a spam database if you take the following precautions:

You can, of course, not publish your Email anywhere at all and not tell it to anyone. But in most cases this is difficult to do, so I advise except the main drawer have one or two minor ones that you will use to register on forums, etc. Often they can also come in handy, which can be obtained without registration at all.
Do not click on links in spam emails (even if there is an “Unsubscribe” button - this is a trap) and, if possible, disable automatic download images in the email client program you are using. There is a chance that your Email will be counted as inactive and mass mailings will not arrive to it in the hundreds per day.
If you have not yet registered a mailbox or are planning to create a new one (for example, because the old one is completely clogged with spam), then do not proceed from the convenience and ease of remembering it, but on the contrary, make it longer and more complex. To your friends, you will still send it to in electronic format, but spammer search programs are unlikely to guess it.

What to do if spam no longer lets you breathe?

These were all preventive measures to combat spam (or rather, reduce its quantity). But there is the possibility of an effective fight even in an already completely neglected situation. In this case, it becomes extremely important.

The fact is that in such large services as or, there are powerful antispam filters.

They place all suspicious emails in a separate Spam folder, thereby freeing the Inbox from junk. Yes, there is no perfect spam cutter, and whenever possible, before cleaning, it would be better to view the contents of the Spam folder diagonally to see if it contains legitimate correspondence. But it’s still much easier than digging through all this garbage all the time.

If you have a mailbox on another service, where the spam cutting is poor (for example, as in), then you shouldn’t despair. Get yourself an Email on Gmail or Yandex, and then set up mail forwarding to it from your old mailbox. Moreover, these settings can be made as in the old mailbox (i.e. set up forwarding - shown in the screenshot):

Also, in the new mailbox, you can configure the collection of mail from your existing Emails (the screenshot shows the settings for collecting mail in Gmail):

The same can be said about email client programs. Most of them also have a built-in spam cutter.

But in this case we should not forget that the mail service will have its own spam folder. this can be corrected in the settings of the service or client program).

Good luck to you! See you soon on the pages of the blog site

Blacklists

They are also DNSBL (DNS-based Blackhole Lists). This is one of the oldest antispam technologies. Block mail coming from IP servers listed in the list.

Pros: The blacklist 100% blocks mail from a suspicious source.
Minuses: They produce a high rate of false positives, so should be used with caution.

Crowd control (DCC, Razor, Pyzor)

The technology involves identifying mass messages in the mail flow that are absolutely identical or differ only slightly. To build a working “mass” analyzer, huge mail flows are required, so this technology is offered by large manufacturers who have significant volumes of mail that they can analyze.

Pros: If the technology worked, then it was guaranteed to detect a mass mailing.
Minuses: Firstly, a “large” mailing may not be spam, but quite legitimate mail (for example, Ozon.ru, Subscribe.ru send thousands of almost identical messages, but this is not spam). Secondly, spammers know how to “break through” such protection using intelligent technologies. They use software that generates various content - text, graphics, etc. - in every spam letter. As a result, crowd control does not work.

Checking Internet Message Headers

Spammers write special programs to generate spam messages and distribute them instantly. At the same time, they make mistakes in the design of headers; as a result, spam does not always comply with the requirements of the RFC mail standard, which describes the header format. These errors can be used to identify a spam message.

Pros: The process of recognizing and filtering spam is transparent, regulated by standards and quite reliable.
Minuses: Spammers are learning quickly, and spam header errors are becoming fewer and fewer. Using this technology alone will allow you to stop no more than a third of all spam.

Content filtering

Also one of the old, proven technologies. The spam message is checked for the presence of spam-specific words, text fragments, pictures and other characteristic spam features. Content filtering began with the analysis of the message subject and those parts of it that contained text (plain text, HTML), but now spam filters check all parts, including graphic attachments.

As a result of the analysis, a text signature can be built or the “spam weight” of a message can be calculated.

Pros: Flexibility, ability to quickly fine-tune. Systems running on this technology easily adapt to new types of spam and rarely make mistakes in distinguishing between spam and normal mail.
Minuses: Updates are usually required. Filter settings are carried out by specially trained people, sometimes by entire antispam laboratories. Such support is expensive, which affects the cost of the spam filter. Spammers invent special tricks to circumvent this technology: they introduce random “noise” into spam, making it difficult to find spam characteristics of a message and evaluate them. For example, they use non-literal symbols in words (this is how, for example, the word viagra may look when using this technique: vi_a_gra or vi@gr@), generate variable colored backgrounds in images, etc.

Content filtering: Bayes

Statistical Bayesian algorithms are also designed for content analysis. Bayesian filters do not require constant tuning. All they need is prior training. After this, the filter is adjusted to the email topics that are typical for this particular user. Thus, if a user works in the education system and conducts training, then personally messages on this topic will not be recognized as spam. For those who do not need offers to attend training, the statistical filter will classify such messages as spam.

Pros: Customization.
Minuses: Works best on individual mail flow. Setting up Bayes on a corporate server with heterogeneous mail is a difficult and thankless task. The main thing is that the end result will be much worse than for individual boxes. If the user is lazy and does not train the filter, then the technology will not be effective. Spammers specifically work to bypass Bayesian filters, and they succeed.

Greylisting

Temporary refusal to receive a message. The refusal comes with an error code that is understood by all mail systems. After some time they resend the message. And programs that send spam do not resend the letter in this case.

Pros: Yes, this is also a solution.
Minuses: Delay in mail delivery. For many users, this solution is unacceptable.

Introduction to the problem

We all know what spam is because we have either encountered it or read about it. We all know how spammers collect email addresses. It is also no secret that spam cannot be completely defeated. The problem is how to maximally protect users who leave their contact details on your website with minimal effort.

Previously tested methods of protection

The biggest threat mailboxes represent programs that download websites and take postal addresses from the text of pages. They either download only your site, or wander around like search engines, throughout the network. If your site is small, the following text auto-replacement protection is quite sufficient:

]+href=)([""]?)mailto:(+)()@".
"()(+.(2,4))2([ >])~i", "1"mailto: [email protected]"
onMouseover="this.href="mai" + "lto:3" + "4" + "%40" + "5" + "6";"7", $text); ?>

Unfortunately, it won't work if you have a large site. Let's say spectator.ru, whose author was one of the first to use this method. If I were a spammer, I would go into personal settings, check the “do not show ears” checkbox, 1000 reviews on the page, and catch cookies with Proxomitron. Then, using a rocker or a PHP script, I would download pages with comments (substituting cookies with settings) and using regular expression I caught the addresses. I would get a small base for advertising mailings.

There were a couple more protection methods in which the mailto: link was automatically replaced with some other one, but the effect remained the same - when you clicked on it, the system client would create a letter to the desired address. Both of them did not stand up to criticism.

Meet the hedgehogs

Obviously, it is difficult to come up with another method of protection other than what has already been tested - providing a form on the site for sending a message. Let's start designing it. The advantages of this method are obvious: no one will be able to get addresses for their spam database from your website. It will not be possible to send messages by hiding your address, as spammers do - the web server will record its IP address. Lists of public anonymous proxy servers are regularly updated, and it is easy to block access from them.

Form sender

Let's start with this, because this is the most difficult part.

When installing a form sender on a site, it is important to protect it from hooligan attacks, which can be no easier than spam. Therefore, we will have to make great efforts in this direction.

First, let's protect ourselves from stupid people double taps and sending many identical requests. The idea is this: the message will not be sent if the user has not previously opened the page with the form, and by opening the page with the form, the message can be sent only once. This can be done using sessions built into PHP. When opening a page with a form, we will launch a session in which we will save a variable, say $flag. We will display the session ID as a hidden element at the very end of the form. The user enters a message and submits the form. Upon receiving the form, the script starts a session and checks the presence and value of the $flag variable. If the variable does not exist, then this is a repeated click, the letter is not sent and an error message is displayed. If the variable exists, and the form data suits us (the required fields are filled in), the script sends a letter and deletes the session.

Secondly, let’s protect ourselves from smart hooligans by recording message logs. If the user submits a correctly filled out form, the script will look at the logs and check what is there. Yes, it should be banned

* send messages to the same address more often than a certain period
* send the same text to different addresses
* and simply use the form sender too often - say, no more than 10 messages per day per user

We display the session ID at the very end of the form, so that the hacker would need to download the entire form and parse it, which is more complicated than simply sending HTTP requests. Naturally, the form sender will issue messages about errors in writing the message, a requirement to indicate a return address, etc.

The resulting form sender code turned out to be too large to include in the text. It has been archived on the website. It seems that the script is working and sending messages.

Replacing addresses in text

Now the form sender is ready, and you need to replace all emails with links to it. Of course, you shouldn't do this manually. For myself, I wrote a script that automatically replaces addresses with links to the form sender.

...Disadvantages: more time for arranging links (compensated by the directory of links), the user, when hovering the cursor over a link, does not see what address he will go to. (Dmitry Smirnov, “Ideal author’s project, hypertextuality”)

All the mentioned disadvantages can be easily eliminated if you use code similar to the one I will now describe and show.

There is nothing complicated here; if these are links, then “more time for arrangement” is not required. On my site I use an engine script that is called by all pages, so it’s not a problem to add code to it or call it from it that replaces addresses. Mailing addresses were and are written directly in the text of the pages, but before being displayed to the user they are replaced with the required text. Compiling a database of links or email addresses is not a problem.

So what does an address replacer do? It searches for “mailto:” links in the text, selects addresses from them, sends a query to the database to count (count(*)) how many addresses from those on the page are in a special table. If there are new addresses on the page, then their number will be greater than the query result. In this case, a query is made in which address values are selected, and those already existing in the table are excluded from the list. The remaining list is sent to the table using an INSERT query.

As for ID addresses, in my opinion it is better to use something that a site visitor could not find. Can you imagine the link /email.php?id=10 leading to the form sender? What a temptation to put 11, 12, etc. there. and try sending them all a message. Therefore, I decided to use the md5 hash of the addresses as identifiers. It’s unlikely that anyone will undertake to select the hash. In the case of a directory of links, you can get by with ID, but then you have to select all the values from the database, and replacing addresses with their hashes is much simpler.

A command of the form is executed

]+href=)". "([""]?)mailto:(+@+". ".(2,4))2(.*?>)~ie", ""12"/email.php ?email=". urlencode(md5("3")). ""4"", $text); ?>

...which replaces addresses with their hashes. I did not dare replace the remaining addresses in the text with links, but made a simple replacement with addresses like vasya_at_pupkin_dot_ru. The autoreplacement code is also in the archive.

Bottom line

Hiding email addresses from visitors is quite easy. The autocorrect mechanism does not require additional effort, and you can continue writing site pages as if nothing had happened. Difficulties arise when protecting the form sender from web hooligans. This protection requires a lot of effort and complex code, so I have not yet used written code on the site. You can download an archive with an address substitute and a form sender, but I ask you very much: do not put it on your site in the form in which you downloaded it, I myself don’t know how reliably it works.

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