I just upgraded to a new HP portable running Windows XP.
I installed Office XP and set up my email account exactly
as it was set up on my previous Windows 98/Office 2000
computer. I can receive emails, but can't send. The
SMTP server times out. I have double-triple-checked the
settings, and talked with AT&T to find the problem.
Everything is set up exactly right. I tried creating a
brand new account and it still fails. I tried using
Outlook Express instead of Outlook, and that works. Same
settings. I can still send emails with my old Windows
98/Office 2000 system, and from my Windows XP/Office 2000
system. I found a link to a LinkSys page about this
problem, and set the MTU to 1492 as it specified. That
also didn't correct the problem. Any help would be
appreciated!

I found that SP3 solved this problem for me! However, I
had problems installing SP3, and had to do the full 58 MB
download. But my email is now working! Maybe SP3 will
solve the problems of some of the other people who have
posted here with problems sending email.