DNS: Is it valid to have an "MX" record and no "A" record?

Solution 1:

As long as the system pointed at by the MX record has an A record itself, then yes.

For example: 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 example.com.

Strictly speaking, 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 example.com.

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 [email protected].

Solution 2:

NO. The MX record points to a name. The names must be resolvable (via A record). The MX record should never point to a CNAME (RFC 1034 section 3.6.2, RFC 1912 section 2.4)

Solution 3:

It is possible - if mail over IPv6 ONLY is desired - as the AAAA record pointed at by the MX record satisfies the address target requirement. Granted, IPv4-only hosts won't be able to contact such a setup, but that doesn't make it illegal under DNS rules.

Why were all the prior answers so IPv4-centric?