Comment by kragen

Oh wow, really? I didn't look because I assumed mail over FTP was transferred over a separate data connection, just like other files. Thank you!

And yes, in August 01972 probably nobody at MIT had ever used ed(1) at Bell Labs. Not impossible, but unlikely; in June, Ritchie had written, "[T]he number of UNIX installations has grown to 10, with more expected." But nothing about it had been published outside Bell Labs.

The rationale is interesting:

> The 'MLFL' command for network mail, though a useful and essential addition to the FTP command repertoire, does not allow TIP users to send mail conveniently without using third hosts. It would be more convenient for TIP users to send mail over the TELNET connection instead of the data connection as provided by the 'MLFL' command.

So that's why they added the MAIL command to FTP, later moved to MTP and then in SMTP split into MAIL, RCPT, and DATA, which still retains the terminating "CRLF.CRLF".

https://gunkies.org/wiki/Terminal_Interface_Processor explains:

> A Terminal Interface Processor (TIP, for short) was a customized IMP variant added to the ARPANET not too long after it was initially deployed. In addition to all the usual IMP functionality (including connection of host computers to the ARPANET), they also provided groups of serial lines to which could be attached terminals, which allowed users at the terminals access to the hosts attached to the ARPANET.

> They were built on Honeywell 316 minicomputers, a later and un-ruggedized variant of the Honeywell 516 minicomputers used in the original IMPs. They used the TELNET protocol, running on top of NCP.