remove, purge, stuck, defer, defered
Postfix: delete mail that isn't getting through
Here is the command:
/usr/sbin/postsuper -d ALL deferred
According the qmgr man page deferred mail is:
Mail that could not be delivered upon the first attempt. The
queue manager implements exponential backoff by doubling the
time between delivery attempts.
So be careful. Maybe there is mail in the deferred queue that
has only been there a short time that will soon get through.