check_postfix_mailqueue is a Nagios plugin which generates statistics for the postfix mailqueue (perfdata) and checks for corrupt messages.
The following values will be checked:
maildrop: Localy posted mail
incoming: Processed local mail and received from network
active: Mails being delivered (should be small)
deferred: Stuck mails (that will be retried later)
corrupt: Messages found to not be in correct format (shold be 0)
hold: Recent addition, messages put on hold indefinitly - delete of free

You can set warn and crit levels for deferred mails.
When critical and warning levels are not set, the plugin returns OK, as long as there are no corrupt messages.
PNP-Template is also available.