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If you are a heavy or business email user, watakoola's suggestion looks good.

Best way to back up mail on a Mac which is for personal use:
1. Get an external drive.
2. Get either CarbonCopyCloner or SuperDuper
3. Use either to create a bootable cloned backup of your internal drive.

Now... EVERYTHING will there, in finder format, and the drive is bootable, too.
EVERYTHING.
 
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