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

Question

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

Answer

Such functionality is not implemented yet, you can vote for it on Plesk UserVoice:

https://plesk.uservoice.com/forums/184549-feature-suggestions/suggestions/5733652-smarthost-external-smtp

The top-ranked suggestions are likely to be included in the next versions of Plesk.