Where are Mac Mail mailboxes located?

youruser/Library/Mail/V2/

You can get to your user's Library folder, which is hidden in Lion, by holding down the Option key and using the Go menu in Finder.

You probably could move your mail folders to an external drive via a symlink to that Mail folder, but I've never tried it. Should anything cause your external drive not to be mounted, it will break Mail, so I don't recommend it. But if you want to try it anyway, here's a basic tutorial on symlinks:

http://gigaom.com/apple/how-to-create-and-use-symlinks-on-a-mac/

Regards

It's keeped in your Library file which is now hidden in Lion.

To see, from the Finder, hold down the option key and choose Go>Library.

Now open the Mail folder. That where you'll find all your mesaages, mailboxes etc.

Unfortunately, as far as I know Mail isn't really designed to keep data in alternate locations like iTunes, iPhoto and iMovie can.

Matt

These instructions are for Lion only. Back up all data first and be prepared to restore if anything goes wrong.

Select  ▹ System Preferences ▹ Users & Groups. Click the lock icon and authenticate. Right-click or control-click your name in the account list and select Advanced options... from the contextual menu. In the sheet that opens, change your home directory to where you want it to be. Save your changes, log out, and log back in.