A while ago I started having a lot of problems with one IMAP account I have configured in mail.app on my mac.
I took the standard approach to delete the account, re-create it, and so on. You can see from prior posts, I ran cli maintenance on the sqllite3 database that mail.app uses.

I got fed up and decided to try something, I would delete the Envelope Index file. The file was around 30mb in size, not huge, but decent size.
When I started mail.app back up, it had to understandably rebuild each of the mailboxes I have configured. When it finished, it is now only 10mb in size, you think there might have been some corruption?