In addition to the standard, mail clients, mail servers and robots can add their own headers, starting with “X-“ (for example, X-Mailer, X-MyServer-Note-ok or X-Spamassasin-Level).

Encrypt mail

To encrypt the mail, two standards are currently widely used: S / MIME (using the public key infrastructure) and OpenPGP (using certificates with a trust scheme grouped around the user).

Commercial use

At the moment, there are the following models of commercial use of mail systems:

Home and corporate mail systems – operate on the owner’s own or leased equipment of the mail system owner (usually he is also the owner of the domain in which the mail server operates).

The service of receiving / sending e-mail is performed by a third-party organization. The organization (person) owns the domain and independently keeps the archive of correspondence.

The service of receiving / sending and storing mail is performed by a third-party organization at its facilities. The customer gets access to the system of the executor for sending letters and for access to the archive of letters. The mail domain is owned by the customer.

Reception, sending, storage of letters is carried out by the executor, the mail domain belongs to the executor. Most of these services are free and work by displaying ads to the user or are a free addition to other services of the performer (for more details, see: Free mail services).

Mailing lists

The mail system allows you to organize complex systems based on sending mail from one to many subscribers, these are:

Mailing lists – a letter from one address with the same (or changing by the template) content, distributed to subscribers of the mailing. Technically, it can be organized as sending a lot of letters (used with templates) or sending a message with many recipients (in fields TO, CC, BCC). To manage large mailing lists (more than 10-50 subscribers), specialized programs (for example, mailman) are used. Correctly organized mailing should control the return of letters (messages about the impossibility to deliver a letter) with the exclusion of unavailable recipients from the mailing list, allowing subscribers to unsubscribe from mailings. Unwanted mailings are called spam and significantly complicate the functioning of mail systems.

Groups of correspondence – a specialized type of mailing, in which a letter to the address of the group (the usual postal address, processing the mail of which the specialized program is engaged) is sent to all the participants of the group. It is analogous to news conferences, echoconferences. Correctly configured mailing should control the cycles (two mailing robots that are subscribed to each other can create an endless cycle of sending letters), restrict the list of mailing participants entitled to post messages, and fulfill other requirements for mailing.

Examples of e-mail management programs:

· mailman;

· Sympa;

· Majordomo.

Spam- is a kind of mailing for the purpose of advertising (often unwanted) a particular product or service, an analogue of paper advertising, distributed free of charge through the mailboxes of residential buildings.

The most popular email services

Most popular e-mail services are provided by IT companies along with other web products: search engines, cloud data stores, etc. The most popular [source is not specified 149 days]

Russian-language e-mail services are developed by Google (Gmail), Yandex (Yandex. Mail), Mail.ru (Mail.ru), Microsoft (Hotmail and Outlook).]