Leigh,
I was only able to recover about 3 or 4 messages from the last two backups - but some went back to last year, so I guess that's all of them.
AFAIK there's no auto delete setting, but it is easy to accidentally hit the "Select All" button near the "Delete" button in the mailbox. A few people have done that in their time.
I recommend you set the "Email Notification" preference to "Yes" in your profile. That way you get an email copy of any PM you receive. You can't reply to them directly of course, but at least you've got a second copy of it somewhere should you need it. Depending on your email software, it's easy to file these PMs in a folder and run searches across them (if, like me, you never throw anything away and have about 500 of them...
)
cheers
RET