DNS: Is it valid to have an "MX" record and no "A" record?
As long as the system pointed at by the MX record has an A record itself, then yes.
example.com can have a MX record pointing at
mail.otherdomain.com. As long as the name mail.otherdomain.com itself is resolvable to an IP address, this is a valid configuration for
mail.otherdomain.com should be an A record with the IP address in order to be RFC-compliant. But this A record will be in the
otherdomain.com domain, not in
Addressing your example, in order for
[email protected] to be a valid email address,
mail.otherdomain.com needs to be configured to handle inbound mail for