MX record lookup check
The check verify that the MX record with the lowest preference is resolvable.
The exchanger field in the MX resource record points to the owner name of the host responsible for receiving emails. That host name has to be resolvable in order for the MX record to work properly and that mail can be delivered.
Event severities and messages
WARNING
Preferred RR {fqdn} (preference {MX pref}), has no associated IPv6 address.
Your mail domain can't receive emails from MTA sources with IPv6 only implemented. This WARNING message does not affect the overall Availability metric but is here to remind about the ongoing deployment of IPv6 as the successor of IP version 4.
Preferred RR {fqdn} (preference {MX pref}), resolves to {IP}. AAAA lookup for {fqdn} failed with {rcode}.
Your mail domain can't receive emails from MTA sources with IPv6 only implemented due to name lookup failure (see error code) on the associated IPv6 address.
ERROR
Preferred RR {fqdn} (preference {MX pref}), has no IPv4 address.
The MX resource record only resolve to an IPv6 address. This negate the ability to receive emails from the vast majority of Mail Transfer Agents.
CRITICAL
Preferred RR {fqdn} (preference {MX pref}) seems to exist but neither resolve to an IPv4 nor an IPv6 address.
The owner name {fqdn} does not resolve to a valid record type (A and/pr AAAA). This make the the preferred MTA impossible to reach and a secondary MTA will be selected for mail delivery. Unless the secondary MTA use a different source of name resolution (other than public) mails can be trapped in this MTA until the DNS problem is resolved.
Preferred RR {fqdn}, (preference {MX pref}) des not exist.
The owner name {fqdn} does not exist. This will cause the sending MTA to deliver the email to the secondary MTA. Unless the secondary MTA use a different source of name resolution (other than public) mails can be trapped in this MTA until the DNS problem is resolved.
Preferred RR {fqdn} (preference {MX pref}), has no IPv4 address. AAAA lookup for {fqdn} failed with {rcode}.
The owner name {fqdn} does not have an associated IPv4 address and the following query for its AAAA record failed. This will cause the sending MTA to deliver the email to the secondary MTA. Unless the secondary MTA use a different source of name resolution (other than public) mails can be trapped in this MTA until the DNS problem is resolved.
UNKNOWN
Unable to perform check due to input data failure.
When this message appear usually one of the queries leading up to the final query have received an empty or garbled query response. Please report this through the normal support channel.
Corrupt or malformed response from nameserver.
The check received a query response with unreadable data fields. Please report this through the normal support channel.
Name lookup for {fqdn} failed with {rcode}
The check encountered a problem during loop and received an unexpected response. See the code for additional information. Please report this through the normal support channel.
Solutions, tips & tricks
The MX resource records with higher preferences are used as backup MTA's if the MTA with the lowest preference don't resolve or for other resons won't accept mail transfers. Once in a while it is a good idea to verify that the backup MX resource records are resolvable and that the hosts accept SMTP traffic (usually on TCP port 25).