I have a machine (centos 5) with one external ip address. On that machine I have many virtual hosts with a few different top level domain names and sub-domains under those eg.
And so on... A lot of these different sites need to send mail as their corresponding domain (or second-level domain at least).
I need to set up DKIM, SPF (but that's handled on the DNS) and rDNS lookup so the mail gets received. As I understand it, you can only have one rDNS entry per machine. I could set up a server for each top level domain, but I'm thinking that'll get expensive.
What should I do here? Is there a way to have multiple rDNS lookups on one ip address? How do I setup DKIM signing for multiple domain names on the one machine? Are there any services out there that could handle this for me?