Postfix logs all failed and successful deliveries to a logfile.
The file is usually called /var/log/maillog or /var/log/mail; the
exact pathname is defined in the /etc/syslog.conf file.
When Postfix does not receive or deliver mail, the first order
of business is to look for errors that prevent Postfix from working
% egrep '(warning|error|fatal|panic):' /some/log/file | more
Note: the most important message is near the BEGINNING of the
output. Error messages that come later are less useful.
The nature of each problem is indicated as follows:
"panic" indicates a problem in the software itself
that only a programmer can fix. Postfix cannot proceed until this
"fatal" is the result of missing files, incorrect
permissions, incorrect configuration file settings that you can
fix. Postfix cannot proceed until this is fixed.
"error" reports a fatal or non-fatal error condition.
Postfix cannot proceed until this is fixed.
"warning" indicates a non-fatal error. These are
problems that you may not be able to fix (such as a broken DNS
server elsewhere on the network) but may also indicate local
configuration errors that could become a problem later.