[Exim] Using more than one mail server

I use exim to post mail from my dialup account. Usually I will use my ntl account (vsmpt.dtn.ntl.com) for sending mail. Occasionally I will use a freeserve account to send mail (smtp.freeserve.net). The problem at the moment is that I cannot use the freeserve mail server when connected to ntl and vice versa.

Is it possible to set up mail so that the config file can detect which server I am connected to and thus know which mail server to use, or is it just easier to have different config files and switch these depending upon which ppp script is run

Matt