Receiving and delivery of postfix mail

Postfix is the MTA mail transmission agent developed by Wietse Venema under the ibm gpl protocol. It tries to be faster, easier to manage, and safer, while maintaining sufficient compatibility with sendmail. The postfix process is more user-friendly.
I. Process of receiving emails
When the postfix receives a new email, it is preferred for the new email to stay in the incoming queue, and then perform different processing for different situations:

1. For local Emails: the sendmail process is responsible for receiving local emails and placing them in the maildrop queue. Then, the pickup process checks the integrity of emails in maildrop. The maildrop directory permission must be set to that a user cannot delete emails from other users.

2. For emails from the network: The smtpd process is responsible for receiving emails from the network and conducting security detection. You can use UCEunsolicitedcommercialemail to control smtpd behaviors.

3. Mail generated by the postfix process: this is to return undeliverable information to the sender. These emails are generated by the bounce background program.

4. emails generated by postfix: The system prompts the postmaster, that is, the postfix administrator.) problems occurred during the running of postfix. For example, SMTP protocol problems, records that violate UCE rules, and so on .) Instructions on cleanup background programs:
Cleanup is the last process for processing a new mail. It processes the new mail as follows: add the lost Form information in the mail header;
To rewrite the address into a standard user@fully.qualified.domain format for arrangement;
Extract the recipient's address from the re-mail header;
Put the mail into the incoming queue and request the mail queue management process to process the mail;
Request the trivial-rewrite process to convert the address to a standard user@fully.qualified.domain format.
Ii. Mail Delivery Process
Once a new mail arrives in the incoming queue, the next step is to start shipping the mail, as shown in process 3 during postfix delivery. Note: The mail queue management process is at the heart of the postfix mail system. It is associated with local, smtp, pipe, and other shipping agents, and sends a shipping request containing the queue file path information, mail sender address, and mail recipient address to the shipping agent. The queue management process maintains a deferred queue, in which messages that cannot be delivered are delivered. In addition, the queue management process also maintains an active queue. The number of mails in the queue is limited to prevent memory overflow when the load is too large. The MQ administrator is also responsible for returning the emails listed in the relocated table to the sender, which contains invalid recipient addresses.

If the message queue management process requests, the rewrite background program parses the recipient address. However, by default, rewrite only distinguishes between local and remote Email recipients. If an email sends a request to your management process, the bounce background program can generate an undeliverable report for the email. The local process of the local shipping agent can understand UNIX-style mailboxes, system alias databases in sendmail style, and. forward files in sendmail style. Multiple local processes can be run at the same time, but the number of concurrent shipping processes for the same user is limited. You can configure local to deliver the mail to the user's home directory, or configure local to send the mail to an external command, such as the Popular local shipping proxy procmail. In the popular linux release version RedHat, procmail is used as the final local delivery proxy. The remote delivery proxy SMTP process queries an SMTP server list based on the recipient address, connects to each SMTP server in sequence, and sorts the table based on performance. When the system load is too large, several concurrent SMTP processes can run simultaneously. Pipe is the shipping agent for the UUCP protocol. The postfix email is described above.

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