In short, go to Mail → Preferences → Accounts → Advanced → “Keep copies of messages for offline viewing” and select: “Don’t keep copies of any messages”.
I just freed up 15 GB of storage by changing this one setting.
But I didn’t stop there. I then decided to go prying to see what else I could free up. I went into ~/Library/Mail/ and saw that the folder was still 15 GB. So I began checking the size of each folder, and discovered that ~/Library/Mail/Mailboxes/Recovered Messages (for my old work email account) had 9 GBs worth of emails and their attachments. I looked through these messages to see they were mostly old emails with hefty attachments. So I selected all the messages, covered my eyes, and hit Delete. (Naturally I backed it all up just in case. Also, I don’t recommend trying this at home.)
All in all, I just cleaned up 25 GB worth of unnecessary email files that were being automatically saved and archived on my local drive.
I already keep a local Archive folder with the emails I want to keep and the rest I delete. Giving up 25 GBs of disc space isn’t worth the option to have a local copy of all my emails. What you lose here is the ability to do a local search of every single email you’ve ever received or sent. But I rarely am in need of one of those locally archived emails. I am, however, daily in need of that 25 GB.
(Via Chris Herbert. Thank you, Chris!)