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; firstname.lastname@example.org. (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:email@example.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.