For discussion:
I pondered over the exact same issues for a couple of days and even raised some support requests with Mythic. My domains are all registered with 123 Reg rather than Mythic. So, I edited the dns on the 123 Reg side to reflect my MB host but I couldn’t seem to get the dns setup sufficiently to satisfy MX Toolbox.
By way of a trial I installed Sympl on a Bytemark machine and then edited the 123 Reg dns to reflect the Bytemark host and voila we had instant gratification of MX Toolbox requirements in every way. This isn’t my preferred option and sadly it appears I cannot escape Bytemark.
I have a theory that perhaps domains registered with Mythic work seamlessly but not those externally registered.
I think my current setup leverages Bytemark’s content dns system and that’s why it just works?!
Rgds Pete