With all the social sending music features on spotify, an outbox to track what music I've sent and to who, would be a huge help. I'm always thinking how much a friend would love a song and then sending it to them, only to have them tell me, I have already sent them the song in the past.
Having a tracker on the outbox would also be useful, to see if they have listened to the song.
Perhaps having a feature of a notification if you try to send the same song to a friend twice or another message if they have heard the song in the past, even if you have never sent them it before.
The outbox request has been active on the old GetSatisfaction forum for over a year, so I think it's about time it got some serious attention on these new forums.