Postfix installation and configuration tutorial postfix Overview

Postfix is the MTA mail transmission agent developed by Wietse Venema under the ibm gpl protocol. The following section is taken from postfix's official site http://www.postfix.org ):
"Postfix is an attempt by Wietse Venema to provide alternatives to the most widely used sendmail. In the Internet world, most emails are delivered through sendmail. About 1 million users use sendmail to deliver hundreds of millions of emails each day. This is a real surprising number. Postfix tries to be faster, easier to manage, and safer, while maintaining sufficient compatibility with sendmail ."

Postfix Overview 1. Features of postfix

1. postfix is free:
Postfix is intended for a wide range of Internet users, trying to affect most Internet email systems, so it is free of charge.
2. Faster:
Postfix is about three times faster than sendmail in terms of performance. A published PC running postfix can send and receive millions of emails each day.
3. Good compatibility:
Postfix is compatible with sendmail, so that sendmail users can easily migrate to postfix. Postfix supports/var [/spool]/mail,/etc/aliases, NIS, and ~ /. Forward file.
4. More robust:
Postfix is designed to work normally under heavy loads. When the system runs beyond the available memory or disk space, the postfix automatically reduces the number of running processes. When the number of processed emails increases, the processes running the postfix will not increase.
5. More flexible:
Postfix is composed of more than a dozen small programs, each program to complete a specific function. You can set the running parameters of each program in the configuration file.
6. Security
Postfix has a multi-layer defense structure to effectively defend against malicious intruders. For example, most postfix programs can run with lower permissions and cannot access local shipping programs related to security through the network.

Postfix overview II. Overall postfix Structure

Postfix consists of more than a dozen semi-resident processes with different functions, and there is no specific parent-child relationship between processes in these processes. A specific process can provide specific services for other processes.
Most postfix processes are centrally managed by one process, which calls other processes as needed. This management process is the master process. This process is also a background program.
These postfix processes can be configured. We can configure the number of running processes, the number of reusable processes, and the survival time. The flexible configuration feature can greatly reduce the operating costs of the entire system.

2.1 postfix overview postfix's mail queue mail queues)
Postfix has four different mail queues, which are managed by the queue management process in a unified manner:
1. maildrop: Local emails are placed in maildrop and copied to incoming.
2. incoming: Place emails that are arriving or are not found by the queue management process.
3. active: Put the queue management process has opened and is preparing to deliver the mail, this queue has a length limit.
4. deferred: Place emails that cannot be delivered.
The queue management process only retains the active queue in the memory and limits the length of the queue. This aims to avoid the process running memory exceeding the available memory of the system.

2.2 postfix overview how postfix handles mail storms
When a new email arrives, the postfix is initialized. during initialization, the postfix only accepts two concurrent connection requests at the same time. When the mail is delivered successfully, the number of concurrent connections that can be accepted at the same time will slowly grow to a configurable value. Of course, if the system consumption reaches the load that the system cannot afford, it will stop increasing. In another case, if the postfix encounters a problem during email processing, the value will begin to decrease.
When the number of new mails received exceeds the delivery capability of the postfix, the postfix will temporarily stop delivering the mails in the deferred queue and handle the newly received mails. This is because the delay in processing new emails is smaller than that in the deferred queue. Postfix will process emails in deferred when idle.

2.3 postfix overview postfix handling of undeliverable emails
When an email cannot be delivered successfully for the first time, the postfix will post a future time stamp to the email. The Message Queue Manager ignores emails with future stamps. When the time stamp expires, the postfix will try to deliver the mail again. If the delivery fails again, the postfix will attach a time stamp twice that of the last time stamp to the mail, when the stamp expires, it will be delivered again, and so on. Of course, after a certain number of attempts, the postfix will discard the delivery of the mail and return an error message to the sender of the mail.

2.4 postfix overview postfix handling of unarrived destination emails
Postfix stores a list of currently inaccessible addresses with limited length in the memory. This avoids attempts to deliver emails whose destination is the current inaccessible address. This greatly improves the system performance.

2.5 postfix overview postfix Security
Postfix improves system security through a series of measures, including:
1. dynamically allocate memory to prevent System Buffer Overflow;
2. Divide large emails into several parts for processing, and re-organize the emails upon delivery;
3. Postfix processes are not controlled by other user processes, but run under the control of the master resident process. They have no parent-child relationship with other user processes, and all have good insulation.
4. the Postfix queue file has a special format and can only be recognized by the postfix itself.