Another thing, the sytem clears out PM's older than 90 days (I think).
The PM system was introduced to try and reduce the number of requests we were getting to pass messages to other members. They take up bandwidth and disk space and so I limited them to allow only 40 messages to be stored.
Unfortunately, that limit of 40 messages includes all copies and tracking receipts. many people are not aware that once their other folders (Copies of sent messages and tracking receipts) reach the 40 limit they cannot receive any more PM's until they delete some of them. To stop the increasing number of people who were telling us that their In-Box was empty but they still couldn't receive PM's I took the decision to automatically delte all messages that were over 90 days old.
So, if you want to keep your old PM's I suggest you save them locally to your PC. The PPRuNe PM system was never meant to be a fully fledged email system.