It shows all your mail threads. The problem is all you see is the title and who posted in it last. So, if you are like me and you are running a custom league with everyone sending you rosters, you have 30+ threads that say something like "roster for clan league" in the title. If I am the last to reply, then there is nothing in the list of mail threads that differentiate them. I have to click EVERY mail thread until I find the one from that person.
SO, I propose that instead of it saying "by ChrisCMU" under the title (for who sent the last message)...it should list the participants in the mail thread instead (or both if that works). If there are more than fit, it can truncate the list with "..." on the end. This would help A LOT. Right now, I still use chat games a lot because the mail system is such a pain in this way.