MX is not crucial for email, everything falls back to A. I have an email server with no MX in production for years now. Not the most important one, but in use.

MX is not crucial for email, everything falls back to A. I have an email server with no MX in production for years now. Not the most important one, but in use. Edited to add: there is a tiny fraction of providers that would reject the emails from such a setup, but it's really not common and does not include large providers.