Apple Mail and IMAP - Removing local attachments only
On 10.14.5 Mojave.

I just went to

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


And removed directory named V6. This cleared out 16.62GB of memory on my SSD. This would only remove the local e-mails. I don't guarantee it wouldn't break the Mail app (not like if I cared... personally, for me it was a mistake to start using it in the first place).

In the Mail app I changed settings to stop downloading attachments by default but it was too late as there was no way of automatically clearing out already downloaded attachments and I didn't feel like removing them manually one by one using the menu.

Before



After