As per Amigojack:
Outbox = waiting to be delivered (phpBB delivers the messages when the recipient logs in).
Sent = the message has been delivered (but not necessarily read)
What may be missing (for some) is a Read-box .... however, that's flirting with privacy-issues. I see no reason why someone should be notified that I have read the message.
The year is 2192. The British Prime Minister visits Brussels to ask for an extension of the Brexit deadline. No one remembers where this tradition originated, but every year it attracts many tourists from all over the world.