What's with the dead.letter files?

Seems like a lot of people on tilde.club have a `dead.letter` file in their home directories. What is it?

If you write a mail in lets say pine but don't send it
but rather quits pine, it will save the mail in
~/dead.letter so you can send it later.

It's used in every (?) console mail client, being a tradition set in the old mail(1) program. It's in the BSD version (before Net, Open, Free, etc. branched off), and may go all the way back to the version in version 1 AT&T Unix, but I can't verify that. That said, the term itself predates electronic mail altogether--it's used by brick and mortar post offices to refer to undeliverable mail.

Thanks for the details. Very interesting.