MX stands for "M
change" or Mail Server. MX is a type of DNS
Resource Record, which tells SMTP
Senders where to send email for a particular domain (specifically identifying a hostname, which must then have at least one A Record pointing to the IP Address
(s) of the Mail Server(s)). "is not an MX for domainname"
means that the Mail Server IP Address
currently under review by the Parser
is not listed in any A Record for any hostname in any MX Record for domainname, and that it therefore is not a registered Mail Server for domainname. Please see RFC 974 MAIL ROUTING AND THE DOMAIN SYSTEM∞
for more information on the use of MX Records.
Also see List of DNS record types