Email still continues to be a popular communication and collaboration tool especially within enterprises. And Postfix (configurable SMTP server) has withstood the test of time and remains popular.
Postfix needs to be made aware of the domains whose mails are to be delegated. This is achieved through the virtual_mailbox_domains setting in the main.cf configuration file.
Then in the transport configuration file, you need to list out the domains and the locations where the mails have to be delegated/forwarded.
Here's an snippet of a main.cf config file with the relevant change:
virtual_mailbox_domains = domain1.com domain2.com
And the transport file forwarding the emails to Haraka running on port 2525:
domain1.com smtp:127.0.0.1:2525 domain2.com smtp:127.0.0.1:2525
Finally, you restart postfix to reflect the changes.