You don't need to be taught :-) ... however, I'll explain my point of view to the other list members:
PTR records point to one single IP address. If you use IMail virtual domains (all domains running on the same IP), the dns reverse check and helo value both show the main machines IP and name, consequently failing the reverse and helo "antispam" tests for every virtual domain.
That isn't true.
If your IMail server hosts example.com and example.net, both on 192.0.2.25, your reverse DNS entry for 192.0.2.25 may be mail.example.com. In that case, if you connect to another IMail (sending E-mail from either virtual domain), it will pass the reverse DNS test (since 192.0.2.25 does have a reverse DNS entry), and it will pass the HELO test (since both mail.example.com and mail.example.net have A records).
If you want to keep multiple domains on the same IMail server machine, you must dedicate a separate IP address for every single mail domain you are hosting, just to satisfy the fancy tests some smart mail admins might activate. This seems like a waste of address space to me.
It is a waste of address space!
There are a few mailserver admins out there (all using non-IMail products!) that do "crazy anti-spam tests" (such as making sure that the HELO and reverse DNS entry match). You can't do anything about them, except educate them (or laugh at them). Why? In most cases, as soon as you jump through hoops for one, you break someone else's crazy anti-spam test.
-Scott
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