Ever since updating to Mac OS 10.1.5 I had problems sending out emails through Verizon who now requires SMTP authentication when sending out mail. For days I couldn't send out anything larger than around 33k before the everything would just stop and I would get an error message. Entourage and Mail.app would both react the same way.
I called Verizon's Tech support and they incorrectly told me to update my Airport software - "My colleagues are telling me its version 2.0.4." I later found out that Verizon is using an MTU setting of 1492 instead of the standard MTU 1500. You can set the MTU manually in the terminal by typing:
% sudo ifconfig en1 mtu 1492Doing this is only good until you reboot your Mac then it defaults back to 1500. Here's my solution.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020712014842725