When we run out of message capacity, we get an error:
Your Private Message xyz, has not been sent!
abc, has exceeded the maximum number of Private Messages they are allowed to receive.
and if we hit the 'return to..' or back button, we simply lose the message we had posted! Not always do I remember to copy before sending... 😆
So my suggestion is, whenever we get a 'spam error' while replying to posts, or the max error message for the private messaging, our reply should be still retained when we hit the back button.