I rarely read PM's but I saw an extra one had appeared some time ago so decided to investigate. It was a terse abusive missive[/QUOTE
Was it from someone annoyed that you had not read their previous 20 PM's?
I have to sympathise with the OP here although I do tend to follow-up any announcements that a PM has arrived. In general they are extremely useful things I find.
Once in a while one can slip through and be missed - usually if travelling or similar.
On the the other hand in more then 20 years of regular internet use I can't recall receiving an abusive PM.
I've had a few heating exchanges in public forums from time to time but never an abusive PM so far as I can recall.
Frankly I would have no idea how to respond in a way that would be certain to resolve such a problem - other than entirely ignore it.