2. Was back in WW2 when many of the American soldiers' girlfriends, fiances and even wives desired relationships with men who were not half way across the world and whose lives were not constantly in peril. These women sent their men a final letter informing them that they were leaving them and, in most cases, finding themselves another suitor back in the States. "
This is the origin, Calpernia.
They were censored - so the recipient would not be dejected and in despair and unable to fight!
ALL MAIL was handled going and coming, and an actual Human used a heavy black pen to line out anything with details about the war or movements and the like - or that might be upsetting to the military person receiving it.
There was no phone contact - no tv - cell phones - etc. - ONLY SNAIL MAIL to communicate, and Western Union - also censored.
It remained in common usage for any letter about breaking up a relationship - hence, not wanting to ever get a 'Dear John' letter!
I suppose it extends to modern day email in the same way.