Discussion on the working mode of SendMail

&http://www.aliyun.com/zixun/aggregation/37954.html ">NBSP; SendMail is widely used and flexible, it has a variety of ways of working and can act as a variety of roles. SendMail can be used as the mail router, SMTP client, and SMTP service program. However, it does not perform the final mail delivery.

The main role of SendMail is to act as a mail router. That is, it obtains the letter, examines the recipient's address, and determines the best path to send the letter. How does SendMail perform this task?

SendMail can determine some of its own information, such as the current time and the hostname in which it runs, but most of its intelligence is sendmail by the user (the "Mail Administrator") through the profile. CF form to provide. This configuration file tells SendMail exactly how to handle all kinds of emails, which are very flexible and powerful, and seem difficult to understand at first glance.

However, as described above, V8 SendMail has a modular set of configuration file constructs that most sites can use to easily construct profiles and include many examples.

As mentioned earlier, SendMail can run as an MTA because it understands the SMTP protocol (V8 SendMail also understands ESMTP). Because SMTP is a connection-oriented protocol, client programs and service programs (also known as senders and sinks) always exist. The SMTP client can post a letter to the SMTP service program, and the SMTP service program will continue to listen on the computer's SMTP port. SendMail can be either an SMTP client or an SMTP service program. When SendMail is run by MUA, it becomes an SMTP client and notifies the SMTP service program of client SMTP (not necessarily another SendMail program), and it runs continuously when the system boots and SendMail starts in daemon mode. The SMTP port is being monitored for new messages, and SendMail is a service program.

One thing SendMail do is the final delivery of a message. SendMail left the task to other programs. SendMail is a large, complex program that runs with superuser privileges, so there are ways to address security issues that have occurred many times in SendMail's past. The additional complexity of mail delivery is the last question SendMail needs to address.