Many different configurations are possible.
What distribution are you on?
How do you plan on authenticating your users? (ldap, or kopano db in mysql)
You’ll also depend on proper SSL setup. One way to do it, using Letsencrypt.
(Assuming you are on latest current Ubuntu distro)
And lookup postfix letsencrypt guides, there are so many out there… I suggest as a building block to start with initial self-signed certs, once you have smtps auth working proper, then move your way up with letsencrypt.
Valid SSL cert is an important part, especially for Outlook.