many ISPs seem to block outgoing connections to port 25 except to their own mail servers, which is pretty annoying....but it can be evaded in many cases by using port 587 instead of 25 for the smtp server settings in the email client.....I did some tests and it worked fine for me...
the full specification can be found here, it's pretty interesting:
http://www.ietf.org/rfc/rfc2476.txt