myhostname = mail.example.com
virtual_alias_domains = example.com
virtual_alias_maps = hash:/etc/postfix/virtual
transport_maps = hash:/etc/postfix/transport

/etc/postfix/virtual:

example.com                 thisdoesntmatter
mailinglist@example.com     mailinglist@example.blah
@example.com                example

/etc/postfix/transport:

example.blah                     smtp:[blah.example.com]