See http://www.postfix.org/DEBUG_README.html#logging
When Postfix does not receive or deliver mail, the first order of business is
to look for errors that prevent Postfix from working properly:
% 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 is fixed.
* "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 an error condition. For safety reasons, a Postfix process
will terminate when more than 13 of these happen.
* "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.
