Some users may experience issues connecting to mailbox using outlook after moving mailboxes from  Exchange 2003/2007 to Exchange 2010 servers

Users can continue to Connect using Outlook Web App ( OWA).

when user try to connect using Outlook, they get following error message

“ Cannot open your default email folder . Microsoft Exchange is not available . Either there are network problems or the Exchange computer is down from maintenance”.


P.S Before follow the procedure check if user is not affected by throttling policy.

- Go to Exchange management shell Run following cmdlet


Check how many policies are present and check which policy is applied on user mailbox

by running following cmdlet

get-mailbox “mailboxname” | fl name,throttlingpolicy

Check for the value RCAMaxConcurrency = 0-100 [ Valid Range ] on throttling policy applied on user mailbox.

If Throttling policy doesn’t fix your issue , continue verifying the logs on ClientAccessServer


--> CAS Server has a logging information in RCA.LOG , you can find the information regarding client connectivity.

check for following error message

/o=First Organization/ou=First Administrative Group/CN=Recipients/cn=User1

RpcDispatch: Unable to map userDn ,/o=First Organization/ou=First Administrative GRoup/CN=Recipients/cn=User1 to exchangePrincipal (StoreError=UnknownUser)

--> Copy the LegacyExchangeDN reported in RCA.Log

-->Go to ADUC –custom Search – Advanced , Write a query to search for duplicate X500 addresses in Active Directory.

(proxyaddresses=x500:/O=First Organization/OU=First Administrative Group/CN=Recipients/CN=User1)

==> Now you should find user with X500 address , stamped with LEGDN Value of User1.

==> Remove the X500. 

[ P.S Any user replying to old messages for user1 may bounce ]

* Do not disconnect and reconnect the mailbox.