The name MAILER-DAEMON is first changed
to postmaster. Then postmaster is looked up again and changed to bob. Because there
is no entry for bob in the aliases file, the mail message is delivered into bob??™s mailbox.
Every aliases file must have an alias for postmaster that will expand to the name of a
real user.* Mail about mail problems is always sent to postmaster bothby mailrelated
programs and by users who are having trouble sending mail.
When mail is bounced (returned because it could not be delivered), it is always sent
from MAILER-DAEMON. That alias is needed because users might reply to bounced mail.
Without it, replies to bounced mail would themselves bounce.
The five types of lines in an aliases file are as follows:
John_Adams: adamj
xpres: ford,carter,reagan,clinton
oldlist: :include:/usr/local/oldguys
nobody: /dev/null
ftphelp: |/usr/local/bin/sendhelp
You have already seen the first line (it was the form used to convert postmaster to
bob). In the previous example, mail sent to John_Adams is delivered to the user whose
login name is adamj.
The xpres: line shows how one name can be expanded into a list of many names.
Eachnew name becomes a new name for further alias processing. If a name can??™t be
further expanded, a copy of the mail message is delivered to it.
The oldlist: line shows how a mailing list can be read from a file.
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