Apple does not seem to give details about this. If you dont want the mail files to be on your main drive you can change this by creating a symbolic link. Dont worry it is very easy.
Where are all the Mail files on a Mac?
They are located at ~/Library/Mail (~/ represents your home directory — /Users/YOURUSER )
Before doing anything
close your Mail App or Outlook 2011 if you have them open.
To change this to something else you have to first move your mail folder to wherever you want.
The command line command for that is:
mv ~/Library/Mail WHERE/YOU WANT/TO MOVE IT
You can also move your files in finder.
After this you just have to create the symbolic link. Open the Console.
The command is:
ln -s path/to/where/you/moved/mail/ ~/Library/Mail
After this is done you can open up Mail or Outlook and all your files will be there.
If you have any problems with this or any questions use the comment box below. Good luck!