That would be tough to implement in a flawless way, as it's a basically a race condition. Unless there is a lock placed on the other person's mailbox right at the moment that you send, it's always possible someone else is sending at the same time, and thus fills the mailbox. I doubt the vBulletin software implements that level of locking, but who knows. The admins will have to speak up on that.
James - 2011 R41 w/Ikon Handle, Feather, Arko (face lather), TGN Super Silvertip brush, Alum Block, Witch Hazel, Gillette Sun-Up AS
BOTOC - LOSER - SSB - ARKO - TOFLAC-U