If it is impossible to deliver the mail to the sender, a service message with a refusal comes. There are three main reasons for the refusal:

Host unknown – the domain name of the computer (mail server) can not be converted to the address – check the spelling of the computer name;

User unknown – user (mail server) is unknown – check the spelling of the user name;

Service unavailable or Can not send message for … days – the destination mail server does not work (no communication or the mail server software is disabled).

Mail retrieval protocols

After the mail reaches the destination server, it performs temporary or permanent storage of the received mail. There are two different models of working with mail: the concept of a mail storage (mailbox) and a mail terminal.

POP3

In the concept of mail storage, mail on the server is stored temporarily, in a limited amount (similar to the mailbox for paper mail), and the user periodically accesses the box and “picks up” letters (that is, the mail client downloads a copy of the letter to itself and deletes the original from the mailbox) . Based on this concept, the POP3 protocol operates.

IMAP

The concept of the postal terminal implies that all correspondence related to the mailbox (including copies of sent messages) is stored on the server, and the user accesses the repository (sometimes it is also traditionally called a “mailbox”) to view correspondence (both new and archive) and writing new letters (including replies to other letters). This principle is the protocol of IMAP and most of the web interfaces of free mail services. This storage of mail correspondence requires much higher capacity from mail servers, as a result, in many cases there is a separation between mail servers that forward mail and mail storage servers.

Differences

Based on the work of the protocols, they can be divided into two main criteria:

Server performance – in this case, IMAP is more resource-intensive than POP3, since all mail processing (such as searching) falls on the server’s shoulders, POP3 only transmits mail to the client;

Bandwidth of the channel – here IMAP in a prize; POP3 transmits the bodies of all letters entirely, whereas IMAP – can transmit separate parts of messages, for example, only text, and the rest – on request.

Under certain conditions, the mail storage server can be configured for behavior similar to the client: such a server accesses the mail server using the POP3 protocol and picks up the mail to itself. Such solutions are usually used in small organizations, in which there is no infrastructure for deploying full-fledged mail servers; In this case, a local server is used to store mail and the mail server of the provider that provides the service for receiving mail via POP3 (for example, using fetchmail). The main drawback of this solution is the delay in delivery (since the receiving software downloads to the server with some delay) – for example, a POP3 connector from Exchange 2003 Server in Windows SBS does not allow an interval of less than 15 minutes to be set by the configuration interface , since An excessive frequency of checks can cause problems with the load on the mail server. Some mail servers have the means to protect against this behavior.

Structure of the letter

When sending via SMTP, an e-mail consists of the following parts:

SMTP envelope data received by the server. Some of this data may be missing in the message itself. For example, in RCPT TO (envelope to) contains a list of recipients of the letter, while in the letter the recipient may not be specified. This information is sent out of the server only within the SMTP protocol, and changing the protocol when delivering mail (for example, at the destination node during internal routing) can lead to the loss of this information. In most cases, this information is not available to the final recipient, which uses non-SMTP protocols (POP3, IMAP) to access the mailbox. To be able to monitor the system’s performance, this information is usually stored in the mail server logs.

The message itself (in the terminology of the SMTP protocol – ‘DATA’), which in turn consists of the following parts, separated by an empty string:

Headers of the letter. In the header, the service information and the marks of the mail servers through which the letter passed, the priority notes, the address and the name of the sender and the recipient of the letter, the subject of the letter and other information are indicated in the header.

The body of the letter. In the body of the letter is, in fact, a letter message.

SMTP envelope data

The SMTP envelope data contains parameters that are specified by the same commands:

The parameter HELO / EHLO – contains the name (FQDN) of the sending node, or the address-literal of the sending node.

The MAIL FROM parameter contains the sender’s e-mail. The address can be arbitrary (including nonexistent, which is allowed by the SMTP protocol, however, RFC 5321 recommends using either Null Reverse-Path for special messages, or an existing e-mail [8]), but this value is used to generate error notifications (rather than the value from the From field of the message header). This address can also be checked during initial spam check [9] and in other cases [what?]. When sending a message, a regular mail client generates MAIL FROM from the contents of the From field.

The RCPT TO parameter is the most important content of the envelope for delivering mail, it contains the electronic address of one or several recipients. When creating an SMTP envelope, RCPT TO can be used several times. When sending a message, a regular mail client forms a list for RCPT TO from the contents of the To, Cc, and Bcc message fields.

Email headers

The headers of the letter are described in RFC standards:

RFC 2076 – Common Internet Message Headers, includes information from other RFCs: RFC 822, RFC 1036, RFC 1123, RFC 1327, RFC 1496, RFC 1521, RFC 1766, RFC 1806, RFC 1864, RFC 1911).

RFC 4021 – Registration of Mail and MIME Header Fields (registration of mail and MIME header fields).

Headers are separated from the body of the message by an empty line. Headers are used to log the passage of letters and service marks (sometimes they are called clads). In Microsoft Outlook, these headers are called “Internet headers”. The headers usually specify: the mail servers through which the email passed (each mail server adds information about who it received this email from), information about whether this email is similar to spam, information about scanning by antivirus, the level of urgency of the message (can change mail servers). Also in the header is usually written the program with which the letter was created. Since the headers are the service information, most often the mail clients hide them from the user during normal reading of letters, but also provide an opportunity to see these headers if there is a need for a more detailed analysis of the letter. In the event that a message from the SMTP format is converted to another format (for example, in Microsoft Exchange 2007, the letters are converted to MAPI), then the headers are saved separately for the possibility of diagnostics.

Headers are usually added from the bottom up (that is, every time a header is added to the message, it is appended to the first line, before all the previous ones).