An SMTP server or Mail Transfer Agent (MTA) is a mail server for transferring e-mails on the Internet using the Simple Mail Transfer Protocol (SMTP). Here the SMTP server can function as a mail relay or mail end system. A further major function provided by the SMTP server is the option of creating mailing lists and forwarding requests with the aid of aliases. Aliases permit the user part of a local receiver address to be replaced by one or more receiver addresses.
The SMTP server uses the Domain Name Service (DNS) to select a suitable route to an end system.
The SMTP server receives messages either from another SMTP server or from a Mail User Agent (MUA). On the basis of the message’s receiver address(es), the message is transferred via a TCP/SMTP connection to another SMTP server and/or a local Mail Delivery Agent (MDA).
Here the local MDA, which stores the messages in special files (mailboxes), is particularly important. The MUA can be used to further process (read, forward, sort, save, reply to, delete) the messages contained in the local mailboxes. In addition, the MUA enables messages to be created and transferred to the local SMTP server for delivery. Some MUAs can also transfer messages to a remote SMTP server via a TCP/SMTP connection.
Figure 3: Client/server mail communication via SMTP, POP3 and IMAP
The IMAP and POP3 mechanisms enable a client system, especially if it is not equipped with an SMTP server (MTA), to access the mailboxes of the SMTP server system. For this purpose, an IMAP and a POP3 server are installed on the SMTP server system. For example, clients (e.g. Outlook or Mozilla/Thunderbird) which run on a remote system can then read and delete messages in the server system’s mailboxes.
In the case of remote User Agents you must ensure that an account number for accounting a POSIX Remote Login Session exists for the users (BS2000 command /ADD-USER or /MODIFY-USER-ATTRIBUTES POSIX-RLOGIN-DEFAULT=*YES).
IMAP and POP3 differ in the following ways:
With IMAP, the e-mails remain on the server, where they are also backed up.
With POP3, the e-mails are by default downloaded to the client and stored there.
Format of the messages
The format of the messages is defined in RFC 822. Messages consist of the message header and the message text. The header and text are separated by precisely one blank line.
Both the message header and the message text consist of readable characters in ASCII format. The message header comprises multiple declarations which in principle are contained in one line, but to enable them to be read more easily can be distributed over several lines. The declarations consist of a name and a text part whose format is defined by the name. Depending on the name, but also on other declarations, a particular type of declaration can occur precisely once, not at all, at most once, or as often as required in a message header.
Most names are defined in RFC 5322. RFC 1522 defines some extensions. Names beginning with “X” are used for private extensions.
The MIME mechanism enables not only pure text but also binary data to be included in the message text. The MIME mechanism defines additional message header declarations and is described in the RFCs 2045 through 2049.