Shulou(Shulou.com)05/31 Report--

What this article shares with you is about the process of receiving and delivering postfix mail. The editor thinks it is very practical, so I share it with you to learn. I hope you can get something after reading this article. Let's take a look at it with the editor.

Postfix is a MTA (Mail transfer Agent) software developed by Wietse Venema under the GPL protocol of IBM. Try to be faster, easier to manage, more secure, while maintaining sufficient compatibility with sendmail. The process of receiving, sending and delivering postfix mail is more humanized.

I. the process of receiving mail

When postfix receives a new message, the new message stops in the incoming queue and is processed differently for different situations:

1. For local messages: the sendmail process is responsible for receiving local messages and putting them in the maildrop queue, and then the pickup process checks the integrity of the messages in maildrop. The permissions of the maildrop directory must be set so that one user cannot delete messages from other users.

2. For mail from the network: the smtpd process is responsible for receiving mail from the network and conducting security testing. You can control the behavior of smtpd through UCE (unsolicitedcommercialemail).

3. Messages generated by the postfix process: this is to return undeliverable information to the sender. These messages are generated by the bounce daemon.

4. An email generated by postfix itself: prompts postmaster (that is, postfix administrator) for problems during the operation of postfix. (such as SMTP protocol issues, records of violations of UCE rules, etc. ) description of the cleanup daemon:

Cleanup is a process in which new mail is processed as follows: add missing Form information in the header

Arrange addresses to be rewritten into standard user@fully.qualified.domain format

Extract the address of the recipient from the duplicate header

Put the message into the incoming queue and request the message queue management process to process the message

Request the trivial-rewrite process to convert the address to the standard user@fully.qualified.domain format.

2. The process of delivering mail

Once the new message arrives in the incoming queue, the next step is to start delivering the message. The postfix process for delivering the message is shown in figure 3. The related instructions are as follows: the mail queue management process is the heart of the entire postfix mail system. It connects with local, smtp, pipe and other delivery agents, and sends delivery requests including queue file path information, mail sender address and mail recipient address to the delivery agent. The queue management process maintains a deferred queue to which undeliverable messages are delivered. In addition, the queue management process maintains an active queue in which the number of messages is limited to prevent memory spills when the load is too heavy. The message queue manager is also responsible for returning messages with recipient addresses listed in the relocated table to the sender, which contains invalid recipient addresses.

If the message queue management process requests, the rewrite daemon resolves the recipient address. By default, however, rewrite only distinguishes whether the mail recipient is local or remote. If the mail requests you to manage the process, the bounce daemon can generate a report that the mail is undeliverable. The local delivery agent local process can understand UNIX-style mailboxes, sendmail-style system alias databases and sendmail-style .forward files. Multiple local processes can be run at the same time, but there is a limit to the number of concurrent posting processes for the same user. You can configure local to deliver mail to the user's host directory, or you can configure local to send mail to an external command, such as the popular local delivery agent procmail. In RedHat, the popular linux distribution, we used procmail as the final local delivery agent. The remote delivery agent SMTP process queries a list of SMTP servers according to the recipient address, connects each SMTP server sequentially, and sorts the table according to performance. When the system load is too heavy, several concurrent SMTP processes can run at the same time. Pipe is the delivery agent for the UUCP protocol.

The above is the process of receiving and delivering postfix mail. The editor believes that there are some knowledge points that we may see or use in our daily work. I hope you can learn more from this article. For more details, please follow the industry information channel.

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