[Users] Setting the HELO/EHLO name

[Adrian]

In response to the very quick and knowledgeable replies from Ralf and
Paul, I'll ask another question!

I'd like to change the "domain" sent in the HELO/EHLO line of the
envelope.  I see that I can set the "Domain name" in the Advanced tab
of the "Preferences for current account", but that also changes the
domain used in the message-ID, which I'd rather keep unchanged.

Why do I ask this?  Well, I have some strange checks that I do.  I used
to resolve the hostname of a sender's sender's remote IP address to see
if it's (probably) one of mine that I should allow relaying for.  But BT
have suddenly started to assign dynamic IP addresses that have no
reverse DNS record, so I can't.  The second method is to test the
HELO/EHLO string, but changing that is falling foul of my checks for
scammers who pretend to send "from" my accounts, as the MessageId
domain should match the From one.  I can fix that test but thought I'd
ask this odd question first.

Adrian