Is it possible to configure Postfix on Plesk to use Smarthost on per domain basis?

Moved to internal, as it didn't work for a customer. Cross-check before applying!

Note: it is possible to configure Postfix to use Smarthost per domain basis via Postfix configuration. Such customizations were not tested properly and are not officially supported by Plesk, so perform it at your own risk.

1. Connect to the server via SSH.

2. Back up Postfix configuration file:

   # cp /etc/postfix/main.cf /etc/postfix/main.cf.bk

3. Create the file /etc/postfix/relay_map

   # touch /etc/postfix/relay_map

4. Edit /etc/postfix/relay_map, adding the following line:

   Note: substitute example.com with the domain which should use the smarthost, smarthost.com with the Smarthost address. 25 is the outgoing mail port (can be altered, as well).

   @example.com [smarthost.com]:25

5. Run the following command to create a hashed version of relay map:

   # postmap /etc/postfix/relay_map

6. Edit /etc/postfix/main.cf, adding the following line:

   sender_dependent_relayhost_maps = hash:/etc/postfix/relay_map

7. Reload Postfix configuration:

   # systemctl reload postfix

Refer to the Postfix official documentation for more information.

Note: Any customization made in Postfix configuration can be overwritten by Plesk Update (for example, in case of a bug fix, when configuration files are updates), Plesk Upgrade or by the mchk utility, which resets default mail server configuration.