Hi All,

Right now i am in a Testing phase before going in full phase migration of Exchange 2010 to Exchange 2013.
Current Environment 
Exchange 2010 - 3 Multi-role (MBX/CAS/HT) servers
Exchange 2013 - 3 Multi-role (MBX/CAS) and 2 dedicated CAS servers.

I have been facing issue when a user whose mailbox is on Exchange 2013 trying to access the mailbox on Exchange 2010. below are the scenario.

1. Auto Configure the Test 1 (Exchange 2013 Mailbox which already has full mailbox permission on 2010 Mailbox) it will popup for user name password, after entering the credentials it goes ahead and completes the config.
2. When i close and re-open the outlook client for test 1 it will show me credential window, after entering it opens successfully.
3. when i configure  a Test 2 (Exchange 2010 mailbox which has full permission on Exchange 2013 mailbox) it works fine without any issue.

I have checked my Outlook-anywhere config of all Exchange 2013 servers and below is the output.

==========================

ExternalHostname                   : mail.contoso.com
InternalHostname                   : internal.domain.contoso.com
ExternalClientAuthenticationMethod : Ntlm
InternalClientAuthenticationMethod : Ntlm
ExternalClientsRequireSsl          : True
InternalClientsRequireSsl          : True

ExternalHostname                   : mail.contoso.com
InternalHostname                   : internal.domain.contoso.com
ExternalClientAuthenticationMethod : Ntlm
InternalClientAuthenticationMethod : Ntlm
ExternalClientsRequireSsl          : True
InternalClientsRequireSsl          : True

ExternalHostname                   : mail.contoso.com
InternalHostname                   : internal.domain.contoso.com
ExternalClientAuthenticationMethod : Ntlm
InternalClientAuthenticationMethod : Ntlm
ExternalClientsRequireSsl          : True
InternalClientsRequireSsl          : True

ExternalHostname                   : mail.contoso.com
InternalHostname                   : internal.domain.contoso.com
ExternalClientAuthenticationMethod : Ntlm
InternalClientAuthenticationMethod : Ntlm
ExternalClientsRequireSsl          : True
InternalClientsRequireSsl          : True

ExternalHostname                   : mail.contoso.com
InternalHostname                   : internal.domain.contoso.com
ExternalClientAuthenticationMethod : Ntlm
InternalClientAuthenticationMethod : Ntlm
ExternalClientsRequireSsl          : True
InternalClientsRequireSsl          : True

=============================

When i check the outlook setting the security mode for Exchange 2013 mailbox shows me negotiate and for Proxy server setting is shows me NTLM

And outlook setting For Exchange 2010 mailbox shows Security as Negotiate and proxy server setting shows me Basic.

Hello,

On the client machine, change the value of LmCompatibilityLevel to 2.

1. Click Start, click Run, type regedit in the Open box, and then press ENTER.
2. Locate and then click the following registry subkey:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\

1. In the right pane, double-click lmcompatibilitylevel.
2. In the Value data box, type a value of 2 , and then click OK.
3. Restart your computer and see if the issue persists.

Thanks,

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There is no registry by Name LmCompatibilityLevel it's a windows 7 machine with outlook 2010

i tried checking on Windows 8.1 machine outlook 2013 there also i was unable to see.

i tried adding the registry but failed again.

I was able to figure it out. 

My exchange 2010 Outlookanywhere FQDN points to 2010 CAS server, once i changed the pointer to point it to my F5 Loadbalancer which host 2013 CAS server it works fine.