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To do this in Apple Mail:
To do this in OWA click on "Conversations By Date" and select "Size".
Once you're set your emails to sort by largest emails first you can then save the large attachments and then delete the large emails you don't need. 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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Don't forget afterwards that you need to go to Mailbox-> Erase Deleted Items-> In All Accounts... to actually delete the emails and free up your email quota.
In OWA:
Then after you've erased deleted items in all your accounts, you can set your email back to view your emails by date.
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