Hmm.. very good question. I know you can do this for phone calls, but I'm not sure that you are able to do it for e-mails. You might have to find a third party app for that, if one exists.
Only thing I can think of-- Ringo Lite -- which allows setting ringtones by contact. Set up an individual contact, using the email as nickname. Hopefully, the system won't bark at the "@" in the name. Not at all sure that this will work, but worth a try.
