MTA relay with recipient filter?

Question

System administration

Good day, Khabrovites!
There is a mail server inside the network on which postfix spins, which stores a list of users in a mysql database.
The task is to install a mail relay that looks at the Internet and accepts only those messages that are addressed to users from postfix, well, and deliver letters from postfix users to external servers. I can upload a list of users with a cron and in any form deliver it to the mail relay.
If anyone has faced such a task - recommend a solution based on some MTA (postfix, sendmail, exim) or can you tell me the key moment of the config - checking users according to the sql-query list or something else.
If there are non-public answers or guesses-suggestions, I will be glad if you write a private message.

Answer 1

You can make the external server access the SQL of the internal ... well, or even master-slave SQL - slave on the external server.

Answer 2

And why not open the internal server for the Internet?
And what about postfix + transport_maps - then you get such a smtp proxy that will proxy from the Internet to a local server.