myhostname = mail.example.com
mydestination = localhost, localhost.localdomain, localhost, $myhostname

virtual_alias_maps = hash:/etc/postfix/valias.txt
transport_maps = hash:/etc/postfix/transport.txt

/etc/postfix/valias.txt:

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

/etc/postfix/transport.txt:

example                     smtp:[1.2.3.4]

# What happens:
#
# * Mail for anything@example.com is resolved to
# "example@mail.example.com" by valias.txt and
# delivered locally. This is good.
#
# * Mail for mailinglist@example.com is resolved to
# "mailinglist@example" by valias.txt and delivered
# by sending via the SMTP server at 1.2.3.4. Close,
# but I need it to be delivered as
# "mailinglist@example.com".