TCP/IP Tutorial and Technical Overview - IBM Redbooks

Figure 15-4 provides a diagram of how these components interact.
MUA
Host 1
Host 2
MUA
Figure 15-4 The typical mail model
Within this model, sendmail plays the role of the MTA. While most current
implementations of sendmail now employ SMTP (see 15.1, “Simple Mail Transfer
Protocol” on page 556), the application has the flexibility to use any mail
protocol—standard or proprietary.
15.2.2 How sendmail works
MTA
MDA
MTA
MDA
Sendmail uses a queuing system to manage inbound and outbound mail.
Running as a daemon, sendmail listens on well-known port 25 for inbound
messages. Upon receiving a message, sendmail spawns a process to queue the
message up in a queue file. Though the location of this file is implementation
specific, it usually exists as /var/spool/mqueue.
If the message can be delivered immediately, the spawned process does so,
de-queues the message, and then terminates. If the message cannot be
delivered, the process leaves the message on the queue, but still terminates. At
regular intervals, the sendmail parent process spawns a new process to check
Chapter 15. Mail applications 569