What is the proper response code for a system which supports EHLO but
for some reason or another the sysop has it turned off (disabled)?
If you don't support EHLO then 2821 no longer applies, and so you have
to follow 821 where EHLO is an unrecognized command, thus the correct
response is 500.
But why disable EHLO? If there are specific services you don't want to
support, don't advertise them in the EHLO response. By accepting the
EHLO you'll avoid the extra round-trips incurred by the fallback to
HELO.
--lyndon