Every single time I open an email to address to my mom, I see this, and absolutely nothing I've tried will get rid of it:
This is absolutely ridiculous. I don't want to see porn spam in my auto complete addresses! There has GOT to be a way to turn this off!
I've gone so far as to search for and eliminate every trace of every email originating from that address in my inbox, junk and trash, quit and restarted Mail and it's still there.
Update: It looks like I may have finally solved this by some obscure post I found on the web. In short, I had to open the Contacts app and disable "Show contacts found in Apps". It seems there was some kind of heinous loop happening here: Contacts was automatically finding contacts based on the emails I received, and Mail was apparently using my Contacts to auto-fill that address, but there was no setting anywhere in Mail to control this!