Mail Server with Multiple IP Addresses & Domains

[cyberdeath]

Hello everyone,

I had a quick question that I was hoping someone could answer.

I currently am running three domains off one server. One is the "primary" domain where most of the users are. I also, however, have two separate domains that will be used off the same machine. I have also bound three public IP addresses on one interface (same default gateway) and would like each domain to go out of each respective IP (I have setup reverse PTR records for each IP to correlate with the domain).

xxx.xxx.xxx.10 mail.domain1.com
xxx.xxx.xxx.11 mail.domain2.com
xxx.xxx.xxx.12 mail.domain3.com

In the example above, 10 is my main domain where most mail will go out of. But, I would like users who have e-mail addresses at the other two domains to use those IP addresses (instead of the primary one [10]) when postfix sends the message....for spam reasons and so when other servers do reverse lookups, they match.

Any guidance would be appreciated.