mirroring one domain.tld to domain.tld.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@aname.com, THOUGH, some are to remain as user2@aname.com.au

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

the users retrive emails as user@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