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For most users there is no reason to keep everything you recieve in email. The easiest way to maintain low disk usage in your email is to view your emails by size.
To do this in Apple Mail:
If you list this view by largest emails first you can then save the large attachments and then delete the large emails you don't need first. This strategy will give you the most bang for the buck in freeing up the most space in your email for the least amount of work.
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