If you use pop2pipe.cgi you can point to any pop3 server. I don't use e-mail piping so I'm not sure that would work. Setup instructions for pop2pipe can be found in this thread:
http://forum.unitedwebcoders.com/index.php/topic,63.0.html.
For outgoing e-mails, TTX either uses sendmail or any SMTP server. These settings can be modified using setup.cgi.