A quick note to my ISP support team resulted in quick action updating the PTR record for the IP I'm using for my mail server, and the related warnings in the mail logs stopped happening!

I'm glad that happened so quickly and fixed everything without other interaction. Really, once all the IPs, DNS, and configuration files were updated, it should just work. And it pretty much did.

Now I'm pondering separating the other services, as the PTR points to the mail server's host name as configured in throughout the e-mail sundry. The IP is used by other services, and is aliased for other domains. When interacting with e-mail, all the domains are configured to use that host name, so not a big deal, but it sits funny with the PTR pointed to the mail "alias," instead of the server's actual host. I can live with it, because it's quick and easy, and fits with the intent of the reasons for validating e-mail sources.

I'm probably overthinking it. For now, PTR will point to mail, and all the other host aliases will be left to other DNS records.