"When you send a PM, the message goes to your Outbox. It remains in the Outbox until the recipient visits his/her Inbox, at which point it is moved from your Outbox to their Inbox (and into your Sent Messages folder - MR). While the message remains in your Outbox you may edit or delete your post if you like. The message remains in your control until it is received."
From the phpBB userguide: