If there is a message in the outbox that cannot be sent, e.g., because it has a badly formatted email address in the header or something, then the backend will attempt to send that message every time someone calls File | Send Unsent Messages. One of the problems this causes is mentioned in bug 749654 -- the backend gives up on the Outbox after the first failure so the rest of the messages in the Outbox are not sent -- but there is another issue that needs to be addressed, i.e., that the bad message gets sent over and over again. This causes its own set of problems, e.g., one user of my add-on (English not his native language) reported that because Thunderbird kept trying to deliver an invalid message, "my email provider blocked my account for that reasons (many incorrect smtps tries per minutes)." If a message in the Outbox fails to send, the backend should probably mark it bad or move it out of the Outbox or do something else to prevent it from being sent again until the user takes steps to confirm that it is OK. On the other hand, I can envision circumstances in which it shouldn't do this, but rather should in fact attempt to deliver the message again. Perhaps the behavior should be configurable, or perhaps it should count how many failures there have been and only give up after a preset number of them.