Postfix is the MTA component used by most linux incoming gateway/mail servers (could be Exim or the old sendmail).
Rspamd is an antispam engine, a very efficient alternative for SpamAssassin.
We are using fixed lists that are periodically updated or updated on demand by our control panel
Because of this, the gateway does not need to query the backend servers every time a mail is processed to check if the recipient is valid and therefore avoid unecessary load on them.
This really makes a difference when there are a lot of incoming mails checked per second.
Another advantage is that if the backend server is temp. down, you can continue accept/reject mails that will stay in delivery queue until the backend is back..
You can configure postfix to do callouts or LDAP validation. It would be OK when handling a reasonable flow of mails but can slow down or create unecessary load with thousands target domains.