Outgoing mail not working

My outgoing mail is not going out. It shows in the sent folder but not received. I get incoming mail fine.

What/who are you using as your outgoing mail server?

As discussed in other items, a provider or two seem to be a little over zealous in their anti-spam mitigation (even on that of authenticated users) and are dumping outbound messages that they have accepted, without bothering to notify their customer.