It used to be even more literally so - network mail started off as using FTP to SNDMSG onto a remote system instead of your own. In RFC475, FTP has MAIL and MLFL (mailfile) commands to support this.
I think it's neat that you can still find echoes of this. MAIL worked by just appending to MLFL, separating records with CRLF.CRLF - which is still how Data segments are terminated in SMTP.
Was that before or after UUCP? I know that UUCP carried a command in each message, so you would specify a message body with a tag that says pass it to the mail receiving peogram.
SNDMSG-over-ftp pre-dates UUCP by 5 years, so I think that one's pretty clear-cut.
(Not that I'm claiming anything's original here, ftp & smtp are both in the nwg/ietf family tree, which makes it easy to draw parallels. There's probably 100 other influences.)
There are rules now, but the concept is still almost intact, random people writing to the servers disk - to be later read by someone