Thanks for your comments. That’s also what I’ve been thinking, that it will be the mail server container’s hostname that’s announced in SMTP connections.
Not sure if there are any downsides to pointing RDNS to the container rather than the host OS. As far as I’m aware, RDNS is pretty much only used in regards to email deliverability and shouldn’t impact anything else by pointing it to the mail server container.
I’ll be testing sometime soon and once I determine for sure how I’ll be setting it up I will post a reply here for anyone in the future who has the same questions & stumbles upon this discussion.