My mail.app has been feeling a bit bloated lately, running a bit sluggish, wrong count numbers showing up, etc.   So I went on a the hunt to make it faster and less bloated. 

On your mac goto:  ~/Library/Mail/ and you should see a file called: Envelope Index, note the size of that file.

Now, exit mail.app (command-q)

open Terminal

go to “cd ~/Library/Mail”

and run “sqlite3 Envelope\ Index”

now inside of sqllite, type “vacuum subjects;”  this will vacuum the bloat out of your inbox, for 1 the size of Envelope Index should be a lot smaller, and for 2, your mail.app should be a lot faster!