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About Dale Moore

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I work for Facilities in the School of Computer Science at Carnegie Mellon University.

I have been at Carnegie Mellon University since 1976. I've worked on lots of different systems programming stuff.

The mission of the School of Computer Science Computing Facilities

Some of my best projects

Other tidbits of information.

Domain Name System MX records are not IPV4 addresses


Why usairwaysmobile.com gets it wrong

The Domain Name System DNS is used to lookup the IP addresses of domains like CNN.COM or CS.CMU.EDU . This IP address is used at the packet layer to route network traffic. In addition to address information, DNS can be used to provide other information about a domain, including Sender Policy Framework SPF and Mail Exchange MX information. The MX record is a way of saying, if you want to send email to this domain via Simple Mail Transfer Protocol SMTP you should contact these hosts listed as MX's for this domain. For example, if we were to send email to Harry.Bovik@CS.CMU.EDU , the mail system would look to see if the domain CS.CMU.EDU has any MX records. The MX records for CS.CMU.EDU are

Because CS.CMU.EDU domain has MX records, we must use the information in the MX to contact that domain. If the domain has no MX records, then it is expected that we will use the IP addresses of that domain instead.

When we receive email, it is reasonable to put some basic sanity checks on the domain portion of the envelope mail from address. We implement these sanity checks so that any email non-delivery reports can be sent back. We refuse to accept any email where we have no available path to send information back to the envelope sender.

A common mistake is to use IPV4 style addresses in MX records instead of Fully Qualified Domain Names FQDN . An example of this mistake is usairwaysmobile.com . They may have changed their behaviour since I wrote this page, so, please feel free to check for yourself.

The company USAirways is an airline. They recently started a program where some of their customers can get updates on their mobile devices about flights, delays, and other information. Some of the updates that are sent out as sent via email with a from address of something@usairwaysmobile.com .

The domain usairwaysmobile.com has the following MX information.

The problem is that is not a domain name, it is an IPV4 address. Nor is . The specification for DNS and SMTP are quire clear. The MX domains must be FQDN's.

Because of this misconfiguration of usairwaysmobile.com, the email that they attempt to send is rejected. There are some sites that are forgiving of this problem. Many email sites are not forgiving. Even if CS.CMU.EDU hosts were reconfigured to accept email from domains with this sort of problem, there are many others that would not. Sites that we forward email to would be quite reasonable to reject email with an envelope from domain with this problem.

This is not a problem with our local domain, it is a problem with domains like usairwaysmobile.com that dont have proper MX information.

We could be exceptionally forgiving of others misconfigurations, and bad behaviour, but that approach has helped fuel the spam problems of the last few decades.

A more correct approach in this case would be if usairwaysmobile.com had the following MX information

I have tried several different approaches in contacting someone responsible for this example domain. I have tried many different email addresses multiple times at usairways. I've tried making phone calls. But all of my efforts appear to have fallen on deaf ears (or unmonitored mailboxes). I sincerely hope that they will soon find enlightenment on this small issue.

Update: I seem to have finally gotten a response from someone responsible for usairwaysmobile.com . They promise to look into this and see what can be done.


The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.

Written by Dale Moore (Last updated )