Just thought I would post this, in case anyone is searching Google and unable to find a solution that works (like me).
Basically what happens is that you write your email, click send as usual, and an email just sits there. Other emails you sent before, and send afterwards, still get sent, but that one just sits there, ignored by the send and receive process.
Advice I found was to delete the outbox file, to restart Outlook, to re-open the email and click send again – none of them worked for me.
What I had to do, was drag the email from Outbox to Drafts, then close Outlook. Re-open outlook, open the email from drafts, click send. It will then send first time.