Postfix ????
/etc/postfix/transport
# EXAMPLES
# In order to deliver internal mail directly, while using a
# mail relay for all other mail, specify a null entry for
# internal destinations (do not change the delivery trans-
# port or the nexthop information) and specify a wildcard
# for all other destinations.
#
# my.domain :
# .my.domain :
# * smtp:outbound-relay.my.domain
#
# In order to send mail for example.com and its subdomains
# via the uucp transport to the UUCP host named example:
#
# example.com uucp:example
# .example.com uucp:example
#
# When no nexthop host name is specified, the destination
# domain name is used instead. For example, the following
# directs mail for user@example.com via the slow transport
# to a mail exchanger for example.com. The slow transport
# could be configured to run at most one delivery process at
# a time:
#
# example.com slow:
#
# When no transport is specified, Postfix uses the transport
# that matches the address domain class (see DESCRIPTION
# above). The following sends all mail for example.com and
# its subdomains to host gateway.example.com:
#
# example.com :[gateway.example.com]
# .example.com :[gateway.example.com]
#
# In the above example, the [] suppress MX lookups. This
# prevents mail routing loops when your machine is primary
# MX host for example.com.
#
# In the case of delivery via SMTP, one may specify host-
# name:service instead of just a host:
#
# example.com smtp:bar.example:2025
#