On CentOS Linux 5.7, Webmin 1.570, Virtualmin 3.89.gpl GPL. Originally set up with one domain added through Virtualmin. At that point email in and out was working just fine (spamassissin & clamav enabled). When I added additional domains through Virtualmin, they could all send email but none of the additional domains could receive any email from external domains: 554 5.7.1. Recipient Address Rejected. Access Denied. Totally baffled as all recipients were most certainly in the Virtual Domains list. Searching the net did not provide any useful information that I could find. Eventually modified return codes under SMTP Server Options so they were not (almost) all 554 - set to the RFC spec and determined that it was 575 5.7.1 (hey, why not set these to the RFC spec in the first place - should I open another issue for this?). From there more searching and a lot of trial and (most definately) error eventually led me to understand that the setting "permit_auth_destination" needed to be added to the Restrictions on recipient addresses.
It seems to me (<1 year with Postfix, 15 years with Sendmail) that if someone is adding multiple domains to a server and if one or more domains are set up with email then the setting that allows them to receive email should be automatically added to Postfix.
There could be something else wrong that was supposed to emable the domains to receive email - but I had not made any Postfix changes directly, only those that might have been by whatever I was doing through Virtualmin - and I could have done that would have removed this setting.
Not sure if this is a "bug" or feature request - I called it a bug but understand if it needs to be changed.