About a week ago (and I can't associate the timing with anything in particular that might have changed on my machine) OL2K started occasionally having an error when sending. The message is something along the lines of "The message couldn't be sent because one of the recipients was rejected by the server. Server response 550 <the email address>...Relaying denied-covad.net customers must check mail before trying to sending mail.
OL2K has stopped blocking messages when I either 1. delete the messages and reenter, 2. ctrl, alt, del, 3. do a hard reboot. Can't find a rhyme or reason. Talked with my ISP and while I was on the phone (and they were telling me how they didn't have a way to fix it) the problem went away.