Objective 108.3: Mail Transfer Agent (MTA) Basics
The most common MTAs on Linux are sendmail, postfix, qmail, and exim.
sendmail, being the oldest MTA, has influenced current MTAs greatly. The other three popular MTAs all have sendmail “compatibility programs” to enable them to act as drop-in replacements for sendmail.
The file /etc/aliases stores aliases for inbound mail addresses; it can redirect mail to one or more users.
Whenever /etc/aliases is modified, newaliases must be executed.
Each user can forward his own mail using a .forward file, containing the forwarding email address, in his home directory.
Outbound mail that is trapped due to a network or other problem will remain queued; it can be examined using the mailq command.