|When mail won't actually go out I find it useful to:|
Check the outgoing server is correct... Check the date on your computer - as Outlook cannot send messages with a date of creation in the future. Once inadvertently I left the date on my PC at a month head (had been checking a day/date combination) and no emails would leave until date was reset correctly.
However you appear to be having a situation I have had a couple of times.
The message(s) simply disappeared - and didn't arrive either - when the "send" button was used. Mail went fine by the same account using the pop-3 direct (online) access for the BT (in UK) account in question. It was only with Outlook 2011 (Mac Office) that the problem occurred. Then "mysteriously it resolved itself... At the time BT was (as per) playing around with their servers and causing all manner of problems for the pholks "out there"... Seems they are still engaged in marrying their systems with Yahoo or is it the other way around?