There is no need to list your mail provider domain names in Postfix.
That just results in unecessary 'user unknown' errors.
Instead, use virtual_alias_maps (NOT: virtual_alias_domains)
mapping of:
user1@emailprovider1.example you@really-local-domain
user2@emailprovider2.example you@really-local-domain
user3@emailprovider3.example other@really-local-domain
Where only really-local-domain delivers to dovecot.
No mail provider domain names in virtual_alias_domains.
No mail provider domain names in virtual_mailbox_domains.
No mail provider domain names in mydestination.
You can even even reverse the virtual_alias_mapping with
smtp_generic_maps, depending on who you are sending email to.
That is a longer story. I can provide that if you are interested.
Wietse
