Further to my previous message, I think what it is is that the secondary
mx doesn't think it is the host listed in dns as secondary MX.
Back when I was running sendmail on it, it was known by multiple names,
stored in /etc/mail/local-host-names. That way it could recognise itself
as being any of the following..
e.g.
mail.myhost.com
relay-1.somewhere.com
mail.thisdomain.com
.. so it didn't matter if the secondary MX for one of my domains was
mail.myhost.com or relay-1.somewhere.com or mail.thisdomain.com, Sendmail
would still accept that it was known by those names and act as secondary
MX as required.
