There are two problems here. One is Mail becoming sluggish and the other is the Mail database becoming too big. It seems natural to think that the performance problem is a consequence of the size problem, but it doesn't have to. It could be that the index needs to be rebuilt, for example, so you may want to try this first:
1. Quit Mail.
2. In the Finder, go to
HOME/Library/Mail/. Make a backup copy of this folder, just in case something goes wrong -- e.g. by dragging it to the Desktop while holding the Option (Alt) key down.
3. Locate the
Envelope Index file and move it to the Trash.
4. Launch Mail again and proceed to re-create the index -- Mail says it's "importing", but it just re-creates
Envelope Index if the mailboxes are already in Mail 2.0 format.
If re-creating the index solves the performance problem, then you know that Mail is perfectly capable of handling your needs, and you just need to find a way to move part of your mail out of the internal HD to solve the size problem.
Mail doesn't recognize Finder aliases, but you can use Unix symbolic links instead. See the following post for instructions to move the entire Mail database to another disk. The same idea can be applied to any folder within
HOME/Library/Mail/ (e.g.
Mailboxes) if you don't want to move the entire Mail folder. Let me know whether you want more specific instructions to accomplish what you want:
Re: Move mailboxes to another disk volume?
Note, however, that Mail would still keep a single
Envelope Index for all your mail, so performance won't improve by moving part of your mail to another disk; this addresses the size problem only.