Dwedit answered the first part of the question. The official phpBB documentation for private messages covers this as well: https://www.phpbb.com/support/documenta ... ser_pm.php
The issue Dwedit describes as "one message stuck in the outbox" may have been mentioned here: viewtopic.php?f=5&t=9181
-- but that said:
This is the first I have heard of a message in the outbox getting "stuck" where the recipient, upon reading it, gets an error of some sort. We need more information on this to track it down please. Chances are it's all related to the phpBB software upgrade.