Easy Fix for that when inbox starts to get full
Save all your messages as a txt file create a folder on your Desktop or USB Stick name it ex. "important PM Msg"
save all pm msg txt files to that folder then empty your inbox without loosing any messages at all txt file will only be a few KB/s you'll have a few hundred text files before you reach even 50mb size
See my pic below that is how i have Zero messages in my inbox without ever loosing anything its an easy fix that any member can always do ..