I use two different SMTP servers, one at home and one at work.
If I try to send an email using a wrong SMTP, ie. the home smtp at work for instance, Mail keeps trying to send it. This can take literally hours for the application to report that it couldn't send the email. And since Mail removes the outgoing email from the outbox, I have no way to change it! Is there a setting in which I can change this timeout time?