After sending a mail from yourself to yourself you can read it for literacy exp. Putting it back in the mailbox and sending it again will allow you to gain the exp again for no cost.
This might be fixed by simply disallowing things to be sent to yourself. Even if it's still possible with two players it's a small enough amount of exp that I don't think it wouldn't be worth the coordination.
It could be worked around by setting mail scrolls as "identified" items. Ultimately, the root cause of the issue is how literacy experience is awarded. A book that someone else has already read, for example, will not grant literacy experience for the second reader.
Fixed in r19852, thanks!