Outgoing messages stuck in Mail's outbox

I just switched a mail account from a POP to an IMAP to sync between an iMac and Macbook Pro. Incoming mail on both are fine, but I'm only able to send mail from the MBP. On the iMac, outgoing mail just sits idle in the Outbox. When trying to send, I get an hourglass icon in the upper-left and "(Delayed)" in the upper-right next to the date and time, which then disappear after about a minute. There's no alert or error message, and the mail just sits in the outbox. Thanks for your help in resolving this issue.