mail client that can store mail in a customised location

I'm having endless problems with Thunderbird and am looking for a new mail client - free or one that costs a small fee.

My main issue is that I want to be able to change the location where mail is stored, so that I can have all the mail in a secure/encrypted part of my mac. I've not found out how to do this in Mail, as it seems to simply re-create a profile folder in the default location. Does anyone know of a client where this is possible?

Postbox looks interesting. It doesn't seem to import Thunderbird data but I'll have a good play with it all the same.

Thanks for the reply.