engine - 11:06 am on Mar 30, 2010 (gmt 0)
Outbox messages are only saved for a short period. The system automatically removes any that are old.
The best option is to forward any messages you want to keep to your e-mail address as the WebmasterWorld StickyMail system was never orginally designed as a storage system.
Oh, and make sure the "Place a backup copy in outbox folder?" box is checked before sending, otherwise, it will not be saved at all.