Remove attachments from sent messages
This will work only on IMAP accounts. Not POP3


Quit Mail.

/Users/&lt;&gt;/Library/Mail/V6 

Navigate there in Finder after turning on hidden files using command+shift+..
Then search that folder for kind:PDF or kind:Image or anything that you think is taking up space.
Delete all those search results. Verify that the folder is taking less space, or alternatively check "About This Mac.app" → Storage → Manage → Trash to see that you've actually saved some space. Empty trash. 
Open Mail Preferences → Accounts → Select your account. Under Account Information, set download attachments to None.