Can't send HTML messages or messages with attachments

[Elizabeth]

Suddenly two computers that I support will not send HTML formatted messages
or messages with attachments. They will send plain text or rich text format.

Running Outlook 2007 on Windows XP, using an Internet POP account. Both
were working fine until today. One is a brand new computer from Dell that I
just installed last Friday. I strongly suspect it was one of the latest
updates that just got installed.

Has anyone else had this problem? Any ideas? I have:

Logged the issue using the new logging function - the system logs into the
smtp server fine.
Completely uninstalled any anti-virus software
The error message we receive is that the connection to the SMTP server timed
out, however, the logs show no problem with the connection. It logs on fine,
it's just when the message is ready to be transmitted Outlook appears to
freeze up.

Thanks in advance for your help!

 

[Elizabeth]

Also, on further investigation, computers running VISTA and Outlook 2007 can
send HTML and attachments. It's only the two Windows XP machines that can't.
And Outlook 2003 works fine also on the Windows 2003 server.

I've gone through every website article I can find remotely relating to
this, and no luck. Any ideas? How does the sending of messages differ in
2007 from 2003?