No MX record? Send anyway

How to get around the lack of a mail exchange record for destinations.

DNS uses MX (mail exchange) records to allow mail delivery. An A record, which delineates a domain name, is not

enough for mail, since most mail delivery is done to a domain rather than a sub domain. Multiple MX records can also be configured to allow multiple send/receive channels for email; most big ISPs have several mail sender / receiver servers to prevent congestion.

Every now and then, a perfectly legitimate address comes along that has no MX record. There are many reasons for this: incompetence on the part of an administrator; perhaps the domain has not been fully configured yet; or it is being transferred from one server to another and hasn't propagated completely.

You can work around this, in Exchange. One way is to temporarily hard-assign an IP address for delivery to the domain name in question. Under the Properties page for the Internet Mail Service, click on the Connections tab, then click the E-Mail Domain button. For each domain to forward to, click the Add button, type in the domain name, select "Forward all messages for this domain to host:" and then type in the IP address that the domain resolves to. Once the domain's MX record is established, you can delete this reference and resort to conventional DNS lookups.

However, if the problem persists for an inordinate length of time, there may be other problems. Some exceptionally paranoid mail servers insist on matching both forward and reverse lookups from all remote hosts. If that's the case, you might need to set up the genuine FQDN for the mail server and set up a CNAME record to point to it for the whole of your domain. This would involve a pair of DNS entries like this:

domain-name.com.     IN MX 10 myserver.real-domain.com.
mail.domain-name.com.     IN CNAME myserver.real-domain.com.

Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter. Check out his Windows 2000 blog for his latest advice and musings on the world of Windows network administrators – please share your thoughts as well!


This was first published in August 2003

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