Does anyone have thoughts on this scenario, please?

You should be able to follow the steps found in the article linked below to get Autodiscover up and running on a mail domain. Once you've confirmed this process works for the first domain, the rest should just be a matter of replicating those changes. 

https://portal.smartertools.com/kb/a3179/set-up-autodiscover-for-smartermail.aspx 

Hi Kyle,

Yes, I did review that article and it works to provide a domain name for POP and IMAP clients along with the ports for this one server. However, I need all my hosted domains to resolve their own MAIL.<uniquedomain>.com domains and ports.

How can I do that?

You should only need to adjust that process to account for your remaining domains. Each domain will require modifications to DNS, IIS, etc. 

Are you talking about SM 16? Version 15 is the one in question.

I have only one option to specify a Host and Port under: Settings > Protocol Settings > POP / IMAP / SMTP In.

Colton, Autodiscover is not configured/setup there in Settings>Protocol Settings. Autodiscover requires DNS settings be in place that direct clients to the correct hostname. Have you reviewed the article I provided previously? 

Yes, I have, thanks. 

The problem is the answer that SmarterMail provides the client. It's one answer for any domain hosted in the SmarterMail environment. I would like the answer for each autodiscover domain to be different, though.

Please let me know how I can do this.

Rats.

Thanks Kyle.