A lot of large sites that have been around for a while have a large accumulation of members who haven't been seen in years.
These members have inboxes full of pm's they never deleted, which is both a security concern as well as a waste of disk space.
It would be great if there were a mod that would expire (delete) pm's once x-amount of time has passed since they were sent or replied to. For example, if you could set a toggle so that once a pm has gone a year since it was sent or last responded to, it would be automatically deleted.