Outlook Web Access: There was an unknown error accessing the directory

While troubleshooting message delivery between two email systems, I received this error from Outlook Web Access:

Message from webpage
There was an unknown error accessing the directory.  If the problem continues, contact technical support for your organization.

I was trying to send a message to contact in my Exchange organization representing a user on the other messaging system. This particular contact was hidden from the address list so I was manually typing the address. I was sending to the targetAddress, or External E-Mail Address, of the contact, not the local address.

A quick web search did not reveal an answer. The only KB from Microsoft regarding the error, http://support.microsoft.com/kb/945917, was not the solution in my case.

First I had a look at the email addresses of the contact. It had only one; userx@sub.domain.com. (domain.com is a shared domain between the two messaging systems.) The address was both in the proxyAddresses and targetAddress attributes. Since sub.domain.com is a sub-domain of the shared SMTP domain I figured I would try adding an address from domain.com. That is when the solution presented itself, because Exchange Management Console (EMC) displayed this error:

Microsoft Exchange Error
The following error(s) occurred while saving changes:

The proxy address “SMTP:userx@sub.domain.com” is already being used by “domain.com/Users/userx”. Please choose another proxy address.

Turns out that there was already a mail enabled user in the directory using the address. That meant that when OWA tried to resolve the address the directory returned two matches. That is one too many. After I resolved this conflict the error disappeared from OWA.

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