Architecture memo of Sendmail and Postfix

Idea of Sendmail + Postfix:

1. They all belong to different SMTP Server implementations.

2. Use the resend mechanism of Sendmail to applyProgramSendmail sent to the local machine will certainly not be lost, and the local machine can connect to the remote Postfix to ensure the use of the resend mechanism. If you directly connect to adm01c, the failure may cause mail loss.

3. If the application directly connects to the remote Postfix, the delay may be caused by Bio. If the application first sends Sendmail to the local machine, it considers that the message has been sent successfully and returns the result. It is similar to sending an email through a message queue. The bottom layer of SMTP is based on the TCP protocol, which ensures sending. So the focus is on how to make the application return quickly.

4. The mail service architecture is as follows: