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.)
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.