mirroring one domain.tld to domain.tld.au

[voytek at sbt.net.au]

I have Postfix/Dovecot/postfixadmin/MySQL with several virtual mailbox
domains

one of the domains is like aname.com.au, the user also now has aname.com,
and, would like to 'mirror' most of the addresses to be user at aname.com,
THOUGH, some are to remain as user2 at aname.com.au

so, both user at aname.com as well as user at aname.com.au should be one user

the users retrive emails as user at aname.com.au

longer term... some would want to use aname.com.au. some, aname.com

what's the best/proper way to do so in Dovecot ?

I think I can do Postfix with postfixadmin to "Mirror addresses of one of
your domains to another."

but what do I do at the Dovecot end...?

thanks for any pointers, suggestions or advice