Every once in a while my postfix mta get clogged with spam and undeliverable mail. This causes wanted email to take a long time to be sent/recieved. In order to resolve the issue, sometimes I get rough with the server and use the following command.
postsuper -v -d ALL deferred
This will basically delete all emails in the deferred queue. So typically, I pray there is nothing I actually want in deferred queue, run the above command, init 6 my mta (just for kicks), and everything then runs fast again.