If a Dear John letter is one written by a woman to the man, saying that the relationship is over, then maybe there could be a response, may be to let the woman know how he felt, and try to get her to return. Of course it wouldn't work if the woman has gone away and not left a forwarding address, and it is not so elegant as an email letter.
Even if the letter was not sent, writing how he felt could help, as it would need some introspection before putting down in words. Just an idea.
Oh, just in case you're wondering, I have not had a 'Dear John' letter before (if I had, I'd probably be