I highly recommend using DKIM and DomainKeys if you will be running your own mail server. I have had mail rejected from services such as Hotmail because I did not sign my emails with these. The Gentoo Wiki has a good article for setup here, but it doesn’t cover virtual hosting1.

For multiple domains, very little needs to be changed. In the domain keys setup, the file /etc/conf.d/dk-filter has the -d flag for domains. More than one domain can be specified via a comma separated list2, such as:

-d domain1.com,domain2.net,domain3.com

For DKIM, the changes are in /etc/mail/dkim-filter/dkim-filter.conf . For the Domain parameter, a comma separated list can be used, like in dk-filter.

Finally, make sure postfix is configured for virtual domains3.

  1. Virtual Hosting is hosting more than one domain on a server. See Wikipedia for more info 

  2. You can also put these domains in a separate file, specified with the -d flag. See the dk-filter man page for details 

  3. I used the Gentoo doc guide Virtual Mailhosting System with Postfix Guide