Yes, this is a good tip. I'd like to add that this is because with no MX record the MTA may still attempt to deliver email to the IP from the domain A/AAAA rec

Yes, this is a good tip. I'd like to add that this is because with no MX record the MTA may still attempt to deliver email to the IP from the domain A/AAAA record. This is a lesser known feature of SMTP, it is specified in rfc5321 section 5: If an empty list of MXs is returned, the address is treated as if it was associated with an implicit MX RR, with a preference of 0, pointing to that host. https://datatracker.ietf.org/doc/html/rfc5321#section-5 https://datatracker.ietf.org/doc/html/rfc5321#section-5