Andrew Pollack's Blog

Technology, Family, Entertainment, Politics, and Random Noise

Here is an obscure mail routing problem I ran into - and the solution

By Andrew Pollack on 10/28/2011 at 05:47 PM EDT


Some mail servers are unable to route to

In this environment, the Windows based Domino servers were affected, but the Linux based Domino servers were not. It turns out that the affected servers are those running Microsoft Windows server operating systems, and of those, only those sites where the local dns domain use a "default" address for misspelled or incorrect server names within the domain. In other words, those where typing would work just as well as www or even It's a valid configuration, but not used in all cases.


The remote site does not have an "MX" (mail exchange) record in their DNS configuration for "".

The Microsoft Windows DNS client lookups use a non-standard approach to name resolution. If a host name is not found exactly as requested, the Microsoft Windows server or workstation attempts to find the server as if it were in the local domain by adding the local domain name to the server named being looked up. That's a fine approach when you're looking for a server address in general, but NOT when you're looking for MX records. This is because it skips a step. First it looks for the MX record for When it fails to find that, what it should do, is look for ANY record for that address as specified. Since a CNAME or an Address record does exist, that IP address should be used. Instead, the Microsoft DNS server skips that and immediately goes looking for a record for "". If "" is using a wildcard default to return misspelled requests to their web server, even though that is NOT an MX record, that address will be returned to the application making the DNS request.

In other words, in the lack of a valid MX record which is an exact match for the request, the Microsoft DNS resolver will return a NON-MX answer that it created itself by adding the local domain to the request AHEAD of a NON-MX record which is an exact match for the request. This clearly backwards -- but not something that Microsoft will likely change, given that they've done it that way for 15 years or so.

A walk through example:

Sending mail from "" to ""

1. The Windows based Mail server attempts to send a message to ""
2. The Mail servers does a DNS lookup for the MX record for ""
3. No MX record exists for ""
4. Microsoft DNS skips doing a lookup to find a NON-MX (A or CNAME) record for ""
5. Microsoft DNS changes the lookup to find a record for ""
6. Sites with default (or wildcard) DNS entries in their domain return that default value in this case (e.g.
7. The Microsoft DNS never goes back to check for a NON-MX record that exactly matches ""
8. The mail server attempts to send the message to the invalid address returned for" and the message fails.
9. Mail isn't delivered.

The behavior is incorrect on the part of the Microsoft server, but that's not going to change any time soon -- it's been the case for nearly 15 years.

What I've done to work around this issue:

Within my own domain, I've had to set up a fake CNAME entry which actually resolves as "" and points to the address of the other server "". This way, when Microsoft does the wrong thing, it gets a workable result -- so the mail goes through.

Obviously what I've done is a wierd workaround and not something that most site admins will be willing (or frankly able) to do.

What the remote side must do to really fix this issue

This issue is probably affecting other people trying to send mail. They would likely be failing, but since they don't have any access to the person who manages their mail servers or those people aren't willing or able to track down an obscure issue like this, those users are simply unable to make it work.

The remote side can resolve this issue, by asking whoever manages their DNS to create a valid MX record for "" If they are using a unix or linux based dns server, it will look something like this:
; ------------------------------------------------------------------------------------------------------------------------------------------------------------ IN MX 100
; ------------------------------------------------------------------------------------------------------------------------------------------------------------

There are  - loading -  comments....

re: Here is an obscure mail routing problem I ran into - and the solutionBy Devin Olson on 10/29/2011 at 08:51 AM EDT
Excellent detective work and analysis. Thanks!

Other Recent Stories...

  1. 09/04/2018With two big projects on hold, I suddenly find myself very available for new short and long term projects. In twenty five years, I don't think I've ever written an entry like this, but if you need the kind of work I do now would be a great time to get in touch. Both of the big projects I had lined up for late summer and early fall have been placed on hold and will be that way for a while. With the kids now all off at college and careers, I'm open to more travel than such than I have been in decades, but unless something else comes along, I'll be here working on updates to Second Signal and other things that ...... 
  2. 07/13/2018Who is HCL and why is it a good thing that they are now the ones behind Notes and Domino?We need to address some biases here. IBM has made a deal under which the Notes & Domino software and intellectual property is now being developed and maintained by HCL America. HCL America is part of the very large "HCL Technologies" company that has grown from its roots in India to become an 8 Billion Dollar company with a global presence in the IT Industry. You could be excused for initially believing, as many people do when they hear this, that "they've outsourced the code to India where they'll milk it ...... 
  3. 03/21/2018Domino Apps on IOS is a Game Changer. Quit holding back.BOOM. This will be as important for the platform as Traveler. If your company has ditched Notes and Domino, I feel sorry for you. For companies that do use Notes/Domino this is a game changer and Apple should be paying attention. Here's why: There are hundreds of little Notes client applications you'd never spend the time and money to build and deploy for your internal user base on IOS that we use Notes for all the time (those of us still using it). Now, those are suddenly ALL available on the iPad. ...... 
  4. 02/15/2018Andrew’s Proposed Gun Laws 
  5. 05/05/2016Is the growing social-sourced economy the modern back door into socialism? 
  6. 04/20/2016Want to be whitelisted? Here are some sensible rules for web site advertising 
  7. 12/30/2015Fantastic new series on Syfy called “The Expanse” – for people who love traditional science fiction 
  8. 10/20/2015My suggestion is to stay away from PayAnywhere(dot)com  
  9. 08/07/2015Here is one for you VMWARE gurus - particularly if you run ESXi without fancy drive arrays 
  10. 08/06/2015The Killer of Orphans (Orphan Documents) 
Click here for more articles.....

pen icon Comment Entry
Your Name
*Your Email
* Your email address is required, but not displayed.
Your thoughts....
Remember Me  

Please wait while your document is saved.