We have an unusual situation. A client has come to us to handle their mailing list emails. They send a substantial number of mails, all are double-opt-in, and fully verified (if they weren't, we'd drop them like a hot rock -- plus, we have severe penalties written into their contract).

What they've run into before is that a provider such as AOL will see a large number of emails from the same IP, assume the mailing was spam, and block the whole thing.

We would like to set up an SMTP server (single machine) to use up tp 20 different IP addresses, and rotate through them in groups, or bursts. For example, a mailing to 1000 recipients would go out as 50 from each of the IP addresses we have assigned to the machine. We have all 20 addresses assigned to the single machine, and the DNS server configured to see all 20 IPs for the same domain name, and (we hope) cycle through them round-robin style, when a request is made to connect with the server to send an email. What seems to not be happening is the rotation by the SMTP server of the IP assigned to the outbound messages.

I am considering implementing multiple virtual mail servers on the machine in question, all with either the same server name assigned (if this will work), or sequential server names assigned (mail1.domain.com, mail2.domain.com, etc.) but don't know if this will work, either.

Can anyone here provide some insight?