Thanks so much for the quick and thorough responses.
Over the past several years we went from a hardened server facility which we sold, to co-locating several critical servers which has worked well. However, having this in house would be less expensive and we are reviewing options.
We are in the midst of moving to a new building which will be about a year out - and currently have a location with consumer fiber, which provides symmetrical 1gb service. However, while they offer static ip addresses for the service, they currently do not offer any dns services, etc.
So - I was hopeful there might some magical combination of ddns and other items that would allow the server to be recognized and accepted in its capacity as a mail server without rdns specifically in place.
So far, based on my research, there is nothing that would match the definitive reference that rdns provides, but know there are some very creative folks out there.
I will keep this updated should I find any alternatives - perhaps benefitting others as well.