I had previously followed the guide linked directly below to complete setup, changing from the default redis port to port 13000.
I have discovered that that guide is not a good example to follow (at least in my instance of setting up a production environment managed using frappe’s supervisor.conf)
The problem was that my supervisor.conf did not include the redis_*.conf files and so was launching everything but the redis server properly. I have no resolved the problem by properly fixing the supervisor.conf file and replacing my old one in the supervisor.d directly.
I am still unable to save a new email domain but believe this is some unrelated error.