Ah - so by default exim accepts mail without *any* HELO command at all.
I hadn't quite realised that. Hey this is going to save me *loads* of
time faking test SMTP sessions and playing with exim -bh etc :)
FWIW, a nearby postfix box is also quite happy with no HELO. By contrast,
sendmail absolutely demands it:
220 mailserver ESMTP Sendmail 8.11.6/8.11.2; ready
MAIL FROM: <>
503 5.0.0 Polite people say HELO first
RCPT TO: <postmaster@???>
503 5.0.0 Need MAIL before RCPT