apt-get install postfix
# Choisir configuration type 'Site internet'

//////////////////////////
//-- etc/postfix/main.cf
//////////////////////////
myhostname = bart
mydomain = mydomain.com
myorigin = $myhostname.$mydomain
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
mydestination = $myhostname, $myorigin, localhost.$mydomain, localhost
mynetworks = 127.0.0.0/8,192.168.10.0/24 # Defines networks allowed for SMTP connections
#luser_relay = user  # used with the local daemon in the transport map,
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
transport_maps = hash:/etc/postfix/transport

//////////////////////////
//-- /etc/postfix/transport
//////////////////////////
extdomain.com smtp:[192.168.10.1]  # relay
.extdomain.com smtp:[192.168.10.1] # tous les sous domaines
* discard # jeter tous les autres messages

//////////////////////////
postmap /etc/postfix/transport
/etc/init.d/postfix reload
# Consulter les logs dans mail.info