"I'll try"  Wow, that was quite some "try"  Kudo's!

> This is important because the original ARPAnet NCP used FTP to do mail transfer.

If you look at RFC 765 (dated June 1980) , there are nearly a dozen mail-related FTP commands. By RFC 959, it's successor five years later, all of them were gone.

Another eary MTA was the MMDF, the Multichannel Memorandum Distribution Facility, from U Delware.  BBN ran it.  One key point is that somehow when used with MH you could get real-time address verification before sending, "user rs@bbn.com doesn't exist." BBN ran MMDF for a long time because a key exec liked/needed that feature, long after 4.2 and sendmail. (My group was one of the first to run sendmail, which I liked because of the "R$" lines in its CF files.  (Not really :) My Usenet/email gateway code had to support MMDF but it was only ever used on bbn.com)

could recognize ArpaNET address postfix and had a hack in it, that allowed the 'user' part of the address to include UUCP addresses

Originally ihnp4!mirror!rs@seismo.arpa, then ihnp4!mirror!rs@seismo.css.gov and then rs%mirror.uucp@seismo.css.gov and then rs@mirror.tmc.com  Thanks to Mary Ann and the "UUCP Mapping Project" for making much of that possible and Peter Honeyman for Pathlias, and MX records for the last part. The pre-MX styles received much scorn from Research, and if you search for "that hideous name" you can find a paper on it. There's also a pathlias paper, which Honey later said "was too good for Usenix" but he had nowhere else to submit it.