EDIT 9/22/2008: The support policy for installing Exchange Server 2007 in an SLD environment has changed, and is now supported. Please go here and read it for follow-up on this situation.

If you attempt to install Exchange 2007 SP1 into an Active Directory forest containing a single-label domain name (e.g. "Contoso" instead of "Contoso.com"), you'll get this error message:

Microsoft Exchange Server 2007 setup cannot continue because this computer belongs to a domain that has a single-labeled DNS name. DNS domain name: XYZ

As the Exchange 2007 SP1 release notes state:

"Exchange 2007 is not supported in environments that use single-label DNS names. Single-label DNS names are DNS names that do not contain a suffix such as .com, .corp, .net, or .org. Exchange 2007 SP1 Setup performs a prerequisite check and will not continue if your environment includes single-label DNS names. For more information about single-label DNS names, see Microsoft Knowledge Base article 300684, Information about configuring Windows for domains with single-label DNS names".

The prerequisite checks built into Exchange 2007 SP1 setup look for this condition and, if found, Setup blocks the installation with the aforementioned error.

While the RTM version of Exchange 2007 also does not support single-label domains, the RTM version of Setup did not block installation into environments containing single-label domain names. With Exchange 2007 SP1, Setup does block installation into a single-label domain. This prereq check is an example of us increasing the amount of things that are checked prior to installations in order to prevent customers from getting into unsupported scenarios. You can expect this trend to continue.

An unintended side effect of the SP1 single-label domain check is that if you installed Exchange 2007 RTM into a single-label domain you are now unable to upgrade to Exchange 2007 SP1. Additionally, attempting to rename your Exchange 2007 domain is not an option because Exchange 2007 does not support domain rename. For some details as to why domain rename is not supported, see the Microsoft Exchange System Attendant service does not start on a computer that is running Exchange Server 2007 after you rename a Windows Server 2003 domain.

What We're Doing about this?

We realize that the new SP1 single-label domain prereq is blocking upgrades. We're actively investigating any issues associated with installing Exchange 2007 SP1 into a single-label domain to see if anything can be done about them. We'll update you on our findings and recommended courses of action on this blog when our investigation is complete. You can expect to hear back from us by February 15, 2008 on this blog.

We also ask that you tell us a little about your environment, if you have a single-label domain and are blocked in your upgrade to Exchange 2007 SP1:

  1. How many domains do you have?
  2. Is the single-label domain a root domain or a child domain?
  3. Does your single-label domain already contain Exchange servers? If so, what version of Exchange?
  4. Do you have Exchange 2000 or Exchange 2003 servers installed in your single-label domain?

You can post replies as comments to this blog post, or if you prefer to keep your answers confidential, please send them to edbeck AT microsoft DOT com. Please use "My SLD Topology" for the subject, if possible!

- Ed Beck

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