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.
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Virtual Hosting is hosting more than one domain on a server. See Wikipedia for more info ↩
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You can also put these domains in a separate file, specified with the -d flag. See the dk-filter man page for details ↩
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I used the Gentoo doc guide Virtual Mailhosting System with Postfix Guide ↩