[GMoromisato]

I don't think kids today realize how little memory we had when SMTP was designed.

For example, the PDP-11 (early 1970s), which was shared among dozens of concurrent users, had 512 kilobytes of RAM. The VAX-11 (late 1970s) might have as much as 2 megabytes.

Programmers were literally counting bytes to write programs.

[NetMageSCW]

I assure you we were not, at least it wasn’t really necessary. Virtual Memory is a powerful drug.

[GMoromisato]

My point is that bytes mattered. If you could put a year in 2 bytes instead of 4, you did. If you could shrink the TCP header by packing fields, you did. And if you could limit SMTP memory use by specifying a 1000-byte limit, then that's what you did.

Every programmer I know from that era knew how big things were in bytes, because it mattered.

Also, not all PDP-11 systems had VM. And the designers of SMTP certainly did not expect that it would only run on systems with VM.