I had to go look this up on the Forumotion support site:
When you send a PM, it immediately goes into the "Outbox". It also immediately goes into the "Inbox" of the recipients. In other words, it's delivered at once.
It remains in your "Outbox" until the recipient actually reads the message, then it moves to your "Sentbox". In other words, you can treat the "Sentbox" as a "Read Receipt" for the message you sent.
If you send to multiple recipients, you get separate entries in the "Outbox" (and eventually the "Sentbox") for each recipients.
All 4 mailboxes have their own quota which is currently set to 50 (or so) which may be enough for most PM interactions.