DNS MX Records error - Loop detected! We were referred back to ‘18.104.22.168'
The MX records were created correctly but when checking from MX Tool Box, an error message is given.
In this case, the Registrar / DNS Provider is Namecheap.
According to Namecheap:
The problem was the NS records were conflicting with the MX records. They had me delete the NS records and it magically started working once the change propagated.