If a mail item on the outgoing mail queue is invalid, it blocks all other emails to be sent.
the Invalid mail item did not have a recipient (to, cc, bcc were empty), and the error that occured out of this did not move the mail item to the error log because the exception is handled incorrectly.
Instead, the invalid item remained on the active queue.
make sure invalid items are moved to the error queue correctly
check that invalid items should be added to the queue at all.