Setup Mail Relay on postfix based on recepient domain

You will need to do all of this as root, so either sudo or elevate to a root shell.

1. Create a file that will contain the domains that will be processed by another server. I used /etc/postfix/bysender.
2. Open the file bysender, add the following information:

   @example.com   [outbound.thirdparty.server.com]
   

   • On the left @example.com is the domain that you want to have its messages forwarded (relayed) to the outside server
   • The right [outbound.thirdparty.server.com] is the server that you want to relay messages to, and yes you need the brackets
   • By the way: You could have one particular user’s messages relayed to a different outside mail server by just putting in the entire e-mail address on the left side
3. Save and Close the bysender file.
4. Now we need to hash/map the bysender file so that Postfix can use it. Run the following command:

   postmap /etc/postfix/bysender

5. Now open your Postfix main.cf and insert the following:

   sender_dependent_relayhost_maps = hash:/etc/postfix/bysender
   

   • Update the file path if needed
6. Now you need to restart Postfix, run the following command:

   service postfix restart 

   • This assumes Ubuntu’s Service Command