This was not helpful.
I have confirmed with my email company that all the settings are correct. That is IMAP for incoming, SMTP for outgoing, Correct box, correct password, SSL off. However, this problem only began at the upgrade to Mavericks. This mail provider still works with my iOS devices (phone/pad- on the same wireless network as the computer- but not at same time) and recieves email fine on my macbook. It simply won't send email The email sits in the outbox. When I check network diagnositcs I get the error message: Trying to log in to thei SMTP account failed. Verify that the username and password are correct. I have verified that both the username and password are correct. My other email accounts (including gmail, verizon, yahoo) are all working just fine. Its very frustrating. In the forums I see other similar comments.