Messaging Server Principles Overview

1, Linux Mail software

1.1 Components of the mail system

The e-mail system in Linux consists of two components: MUA (mail user agent, mail users proxy), and MTA (mail transport agent, message transfer agents). MUA is a mail system for users to read and write the message interface, and MTA test run at the bottom, able to handle the mail sent and received the work of the program. Simply put, the user can use MUA to write letters, read letters, and through the MTA to receive letters, letters.

1.2 Commonly used MTA programs

The most common e-mail MTA system is SendMail, which was initially integrated into an e-mail system developed by the University of California. There are several versions of SendMail in the Linux system, and the differences in these versions are not very large. Another common MTA program is Smail, which was developed by Curt Noll and Ronald Kart.

Since both programs can be found on Linux publishing discs, either is a good choice for small applications. Since Smail is a more modern product, it is slightly easier to configure and use. For large systems or applications with special configuration requirements, SendMail is more flexible and can achieve better performance. Their basic functions in addition to the most basic mail delivery, as well as letter forwarding processing, accumulation (to send) letter processing, different transmission tools to judge and retreat processing and so on.

When we need to use SendMail or smail, we need to customize the package with a configuration file to complete the various operations we need. SendMail's configuration is very complex, and it has a complex configuration file that is complex to the point where manual configuration is difficult. As a result, it is often necessary to run another application to achieve simplicity in the configuration. In comparison, the configuration of Smail is very simple.

1.3 Mua and MTA run together

When writing a letter (such as Elm,pine or mail) with a MUA program, the application passes the letter to an MTA program such as SendMail or smail (in effect, the MUA program passes the letter to a normal server called Rmail, which is usually the alias of the MTA program in the system). Although most systems use only one MTA program, you can actually run several of these programs at the same time, such as running an MTA program to handle LAN mail and running another MTA program to process UUCP letters.

If a letter is sent to a local area network or localhost, the MTA program should be able to determine this information from the address. The MTA program also supports an alias mechanism that allows users to easily communicate with other users, hosts, or networks with different names. If a letter is sent to a remote system user, the MTA program must be able to select routes, establish a connection to the Remote mail server, and send mail. This remote connection can be built on top of UUCP or TCP. If the latter is the case, then the SMTP protocol (Simple Mail Transfer Protocol, Easy Message Transfer Protocol) will be adopted. The MTA program must also be able to handle problems with sending messages, and can report error messages to the sender, for example, when the message is not filled in or the recipient does not exist, the MTA program will make an error to the sender.