It's definitely a great idea. I thought more people had brought it up, but maybe it's something we've all thought about and never really vocalized. Either way, YES! I would love to see this feature.
Yes, yes, yes! I had this same problem after I finished my Agoraphobe experiment and was sending out dozens and dozens of "Thank you" notes. It was a bit of a nightmare trying to keep track of everything.
At Glitchmas, too. And trying to keep track of whether I was the last to send stuff when I have a long-term barter thang going on. And if I ever have to communicate with my entire Resource Route by mail ... oh please let us have a record of what we have sent.
Right now sending mail is a little like being in Memento, and I'm running out of tattoo space.