a few ISPs that I know of take measures to "force" you to use their smtp server while on their network.

The company I work for uses Earthlink dial up for people on travel with laptops and we get calls because users forget to change their outgoing mail server address =/

If this is the case with your ISP, there isn't much you can do other than use the mail server they want you to use or ssh/vpn tunnel around them.