I have read what others have said about mail getting stuck in the Outbox, but my question is why?
I have been operating with the same mail account in Outlook and never had a problem sending mail. In fact, I use 2 separate accounts in Outlook and one does not have a problem. When the sending fails the message says that Outlook 2010 cannot connect to the smtp server. However, after clicking work offline I open the mail and select to send from my other mail account, the mail is sent no problem.
Not to over simplify, but this account is a Gmail account. If I log into GMail I can send from there, but it's just Outlook stops sending for some unknown reason.