Hi there,
We have a dynamically-assigned IP address, linked to a DYNU account, which works perfectly for DNS resolution of a self-hosted website. However, we are struggling with the reverse-DNS record for email services. (we cannot get this service from our current ISP).

"If you are listed in the Spamrats/RATS-NoPtr blacklist and you operate your own mail server, you have no "Reverse-DNS Entry" in place with your ISP to resolve that IP address to your server's hostname."

Does Dynu any reverse-dns service?
Note we host about 30 domains at another ISP, as well as the 1 which currently uses DynU services. These domains can be switched if we can resolve the above problem in a cost-effective manner.

Have you considered hosting the email server on a cloud VPS? You get your own static IP with which you can set up reverse DNS. https://www.dynu.com/en-US/ControlPanel/AddVPS