If you believe that when you "don't see" a PM (or email) you have deleted that it means it's actually deleted and gone for good then you maybe naive about how computer systems typically operate. Most systems use what is called 'soft delete' which means they are filtered out but still very much there and they have to be to manage a site properly.
I am very well aware that rarely anything is truely deleted Simon from forums (and it's not how computer systems operate as if they had a mind of their own.. it's more how computer systems operators operate and it does not make it right) I certainly understand you as the Admin of this forum keeping a backup of everything, I understand your point too regarding people behaving badly with abusing the messaging system. But does that not seem also like a little NSA & Big Brother-ish? Because a minority have misbehaved now every single person that ever sends any message any time for ever will have it saved and recorded for ever. Seems like a case of everyone losing freedoms and rights to their own info because of a few bad apples.
That certainly is not correct.