Obviously those all overlapped a good deal. Your immediate reaction
might be to make sure that the sender domain matches the HELO. After
all, it would nail half the spam right there. But then again, it
would also block most of the mail coming from my domain and many
others. My mail server always uses the primary domain name in the
HELO, no matter which domain it sends for. That's probably true of
most servers.
Yes, we host 300 domains...trying to do RFC checking is a miserable
experience when many mail admins don't even perform reverse-DNS registration
or forward registration of the HELO host.
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