>>NetMag+(OP)
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.