Have a look at the error logs.
http://www.postfix.org/DEBUG_README.html#logging
Look for obvious signs of trouble
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Postfix logs all failed and successful deliveries to a logfile.
* When Postfix uses syslog logging (the default), the file is usually
called /var/log/maillog, /var/log/mail, or something similar; the
exact pathname is configured in a file called /etc/syslog.conf,
/etc/rsyslog.conf, or something similar.
* When Postfix uses its own logging system (see MAILLOG_README),
the location of the logfile is configured with the Postfix
maillog_file parameter.
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.
