Does you connection to the smtp server cross from one internet provider's
equipment to another's along the way? If so, such connections are often
blocked to cause trouble for spammers if they use port 25, and occasionally
if they use some other port. You may want to check if your mail provider
has set this server to listen for outgoing mail on some other port as well,
and
if so, get the details of using the other port. You could also switch to
using
the outgoing email server of the first internet provider in the connection,
but
keep the same incoming email server and the same return address as before.
Also, what if any antivirus or other security programs do you have? Some
of them cause assorted problems in Windows Mail, although seldom
immediately, if they scan email.