Exchange 2010 : Outlook Cannot connect to Mailbox after migrating to Exchange 2010 Server but Can connect using OWA)

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

    get-Throttlingpolicy

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.