> I was going by Wikipedia's dates, which give the actual use of IPv4 on ARPANET as January 1983, which I believe postdates the actual use of SMTP on ARPANET.

> I was going by Wikipedia's dates, which give the actual use of IPv4 on ARPANET as January 1983, which I believe postdates the actual use of SMTP on ARPANET. This is documented on RFC 801 (https://datatracker.ietf.org/doc/html/rfc801 https://datatracker.ietf.org/doc/html/rfc801), which literally just mandated use of the previous NCP until 1982 and then switch to the incompatible TCP/IP protocol stack in 1983. Imagine doing that with IPv4!