How to forward an email to an external domain in postfix (linux):
Configure postfix like a normal mailserver. In the config you can use virtual_alias_maps to create a mapping table for the delivery of the emails.
In this file you can forward every email to the address user@MYDOMAIN.TLD to a other user from an external domain.
apt-get install postfix -> configure as a internet site -> domainname: mail.MYDOMAIN.TLD nano /etc/postfix/main.cf # change myhostname = MYDOMAIN.TLD mydestination = mail.MYDOMAIN.TLD, localhost.com, localhost # insert at the bottom of the file virtual_alias_domains = MYDOMAIN.TLD virtual_alias_maps = hash:/etc/postfix/virtual nano /etc/postfix/virtual user@MYDOMAIN.TLD user@EXTERNALDOMAIN.TLD postmap /etc/postfix/virtual /etc/init.d/postfix restart set the mx record of MYDOMAIN to mail.MYDOMAIN.TLD. set the a record of the subdomain mail to the ip address of MYDOMAIN set the ptr record from the ip address of MYDOMAIN to MYDOMAIN.TLD
