Forward email to external domain in postfix (linux):
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
