RonKolb Said:
"Well, that got rid of another GB, but there’s still 200+ GB in there. Is there a cache that somehow retains everything, as does it need to be emptied?“
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Thank you for the update.
Three Things to Try:
A. Look for Attachments:
Seems to be all attachments and caches. So, see if you can find a way to view which eMail has any attachments.
B. Freeing Caches:
If not, then cache seems to be the issue here. To put it bluntly, you’d backup your Mac, erase the drive, and then restore your Mac from the backup. That would free the caches, if existent.
Or...
C. Reinstall Anew:
If nether of the above work, then back up your Mac > make note of what you have for applications > erase the drive in Recovery Mode > install it anew. You’d import much of it through use of Migration Assistant or just manually transfer it over. The is the site to use: What to do before you sell, give away, or trade in your Mac What to do Before you Sell, Give Away, or Trade in your Mac - Apple Support.