Can an MX record use an IP address?

Question

Can an MX (Mail-Exchange) record be an IP address?

Answer

No.

Using an MX (Mail-Exchange) record, a mail server will not accept it is if is an IP address.

This can be seen in the Wikipedia article for MX records, and well as, RFC1035 Section-3.3.9.

Here is an example of a properly constructed MX record. It is ok to use either a 'domain name', or a 'subdomain name':

Using domain name:

domain.tld. 14400 IN MX 10 domain.tld.
domain.tld. 14400 IN MX 20 domain.tld.
domain.tld. 14400 IN MX 30 domain.tld.

Using sub-domain name:

domain.tld. 14400 IN MX 10 mail.domain.tld.
domain.tld. 14400 IN MX 20 mail.domain.tld.
domain.tld. 14400 IN MX 30 mail.domain.tld.

The MX record name will need to resolve to the proper IP address as well.