Maybe an easier way is to disable all members' ability to delete PM's. That should be an easy hack, but you'd need to auto age off old PM's, prune them periodically, or allow unlimited number of PM's.

I cant think of a feasible way of doing this without using too much resources.

I agree ... I avoid such places.

Im interested in the same thing.

How do you save a copy of all PMs send before they are getting deleted
without having a no limit of PMs for users?

I would love a PM from anyone who could think of a way of doing this!

Cheers