Hi Owen,
As far as I know, messages stay in outbox until the other one reads them.
It's not like email, it's a private message put on the forum database.
You can edit the message as long it's in the outbox, but not when read, then it's a send message.
And Aaron, Mod's can't read them, but it's just that most members don't read there PM box often...
I forget to look there as well once and a while.
Greetz Remco (MOD)