@cbiweb
Another possibility to consider:
(On the basis of your question, I am assuming that you want to keep things off the C:\ drive when possible, and you have a D:\ drive where you keep non-OS stuff when possible.)
Use the
portable version of Thunderbird. You don't even have to uninstall your current (installed) version first. Just try the portable and see for yourself. I have mine "installed" on my D:\ drive, and all of its supporting files are kept right there. You can open the ThunderbirdPortable folder, then open the Data folder, and then open the Profile folder, and you'll see all of your mail and settings and operating files stored right there.
If you decide to try the portable, post here with any questions you have, and I'll try to handle them for you. I'm a D-drive-portable-apps addict, and I've had a bunch of experience with the portable versions of many software programs.
Thunderbird is the safe, fast email client that's easy to use. It has lots of great features including quick message search, customizable views, support for IMAP/POP, RSS support and more. Plus, the portable version leaves no personal information behind on the machine you run it on, so you can...
portableapps.com
If you can get your portables from PortableApps.com, do. There are many purveyors of portables online who are less than scrupulous about what they deliver. Portable Apps is absolutely trustworthy, and their versions are absolutely reliable.