I need to be able to set up the MTA to handle a hostname so that:

I can forward all emails to the host to an external (or internal) email address, excepting specific emails that I can set to return 'nouser' for. Hosts should be easily addable in multiple domains handled by the MTA.

example:
'anything'@host1.domain.net >>-> some1@yahoo.com, 'anything'@host2.domain.net >>-> me@localhost
where spamsource@host1.domain.net gives a 550 'nouser' response.

'anything' addresses need not exist on localhost.

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Ideally the MTA would be adminable through Webmin; but if it's easily handled via text files, I can deal.

Sendmail's virtusertable can do this pretty easily, but I usually admin Sendmail from Webmin, and the Webmin/Sendmail combination seems to be broken on Debian (configurations are forgotton whenever the network card is ifup'ed/ifdown'ed.) As to easily adminning sendmail via text files, well - no. I won't do it.