We have an old zimbra server as the company mailserver, which does TLS1 only. Let's call zimbra for MailServerA.
To fix this (and sort out some security issues), I'd like to setup a postfix server to receive incoming email from the internet. Let's call this server for MailServerB
Then it should only forward email to MailServerA if the recipient's domain is in my list of domains.
How do I configure MailServerB to accept email from anyone, but only relay to MailserverA if the email is for one of my domains?