It may not be 30 days but it's some period of time. I remember going back to look through my trash for a really old email once and it was not available.
Ah, there's the key piece of info (I actually pointed at the screen and said "aaaah" when I read your post because it all began to make sense). My mail doesn't even make it to the trash, I read it and then I forget it and it sits in my inbox where it will sit for all eternity if I do not move it to trash. Every so often I go through and delete emails but generally that is something I don't do because I forget, or I'm lazy, or something (and I think Inky was describing the same kind of activity of his part).