Googling for some answers regarding why some mail-servers fail to identify themselves correctly, I found this amusing forum reply;
Unfortunately, “reject_unknown_helo_hostname” is not acceptable to use, because too many Exchange Servers are configured to identify themselves with something that only appears in their local DNS.
Apparently either Microsoft’s certification programs only produce people who can point and click and don’t know how anything about RFC standards, or their programs don’t allow a proper internet routable hostname to be set on the server for some reason, possibly having to do with clustering or other advanced things used in larger networks.
I’m not sure what it is, but I heard that a few of the issues preventing this sort of thing were removed in Exchange 2007 and 2010, but most Exchange admins will most likely not be aware of them.
It would be nice if this wasn’t an issue, because then we could all reject messages that don’t pass this test. For our tiny network, this would be at least 500 messages a day, which is a good chunk of all the spam we receive. Instead, these messages end up making it to Amavis+SpamAssassin where they waste our CPU time.
Oh well, I suppose it could be much worse…and it usually is with Microsoft Server OSes.