I found a brilliant little gem in the MailMate menus: you can change what MailMate searches by default! For a while now I've been frustrated by the default search because it only searched the "from" field of my emails and I generally just want to search everything. So, I set up my search with to look through the "Common Headers or Body":

Screenshot showing the searchable fields — Common Headers or Body is the way to go

And then hit the button:

Screenshot showing menu item to make a search the default — Set it as the default

Simple enough, but it alleviated some pain for me.