You sure? My understanding of phpBB was that the message stays in the Outbox until the user logs on, at which point it gets sent over and moved to the "Sent" folder, assuming the user has space in their Inbox. I've never heard that it requires them to read it first... Being simultaneously in their Inbox and my Outbox prior to reading doesn't sound logical, especially since the FBTB boards have always had the "your box is too full to receive a message" thing for several iterations.
Anyway, regardless... Just trying to avoid missed messages and whatnot.