I do a lot of development on computers that use different smtp servers when in beta and production. I also need to change my smtp settings in my E-mail client often when I travel. I found a solution to both problems, X-Ray Mail Assistant.
X-ray lets you setup a Windows service that becomes an SMTP/POP replay server on your local machine. You can set it up to forward localhost connections to multiple SMTP servers based on your IP address. Now I can setup all my PHP mail scripts to use localhost and travel without changing my smtp server setting in Thunderbird again.