I was having trouble sending mail with my aim.com account, but fortunately I was able to walk down the hall and talk with the head of mail development, who informed me of the error of my ways...
You must use smtp.aim.com:587 for sending mail, NOT smtp.aol.com. On the IMAP side, you should use imap.aim.com:143, although imap.aol.com will still work. When I made this change, things worked fine for me.
He also mentioned that SSL/TLS is a work in progress, so you might be
able to use SSL/TLS for your aim.com SMTP and IMAP - but you might not.