Technically, MX are not mandatory. As per RFC2821, if no MX is found the A record will be used. In the past I used this fact regularly when obtaining SSL certi

Technically, MX are not mandatory. As per RFC2821, if no MX is found the A record will be used. In the past I used this fact regularly when obtaining SSL certificates for subdomains that were pointed at my server. Customer "foo.com" would point app.foo.com at my server for me to host my service for them on their subdomain. To obtain an SSL certificate, I would select email validation to admin@app.foo.com and not need an MX record in the customer's DNS system in order to receive the email, it would be delivered purely based on the A record pointing to my server.