Hey,

Correct me if I'm wrong, but isn't so that OWA should work on both ways when you have e.g. E2007 in old forest and E2013 on new forest?

I have ForestA with E2007 and ForestB with E2013. Some users are migrated from E2007 to E2013 and mail flows and availability services are working just fine. Also when user is moved they profiles are updated automatically.

But now the OWA added some grey hairs to me:
Case1:
when open OWA using the owa.ForestA.com E2007 users are working, E2013 users gets error:
Outlook Web Access could not find a mailbox for ForestB\User01. If the problem continues, contact technical support for your organization and tell them the following: The mailbox may be stored on a Microsoft Exchange 2000 or Microsoft Exchange 2003 server, or the Active Directory user account was created recently and has not yet replicated to the Active Directory site where this Client Access server is hosted. 

Case 2:
When OWA is opened using the owa.ForestB.com E2013 users are working, but E2007 users get an error:
X-OWA-Error: Microsoft.Exchange.Clients.Owa2.Server.Core.OwaUserHasNoMailboxAndNoLicenseAssignedException

E2013 users have Mail User object in ForestA and E2007 users have Mail User object on ForestB. Also both Mail User objects have the ExternalAddress attribute populated.

I have a feeling that this is based on the Autodiscover config, but I have not figure out which part of

Hi 

In coexistence with exchange 2013 and legacy version the request happens in 2 types.
For Exchange 2010  Exchange 2013 does a Proxy for owa and ews requests for users in exchange 2010.
For Exchange 2007  Exchange 2013 does redirection for owa and ews requests for users in Exchange 2007.

When a user with an Exchange 2007 mailbox logins externally from OWA the requests goes to Exchange 2013. Now the Exchange 2013 needs this connection to be redirected to exchange
2007 server.

In Order to do this Exchange 2013 requires a dedicated external host name configured on exchange
2007 servers for the required services accessed from externally. So the external and internal hostnames of the Exchange 2007 server need to be different from the hostnames of the Exchange 2013 server and need to be pointed to the Exchange 2007 server.

Example:

For Public DNS
Current external owa URL (contoso.domain.com) point it to dedicated exchange 2013 public ip or internet facing exchange 2013 CAS FQDN.
Current External Autodiscover point it to dedicated exchange 2013 public ip or internet
facing exchange 2013 CAS FQDN

Internal DNS  Configure the Exchange 2007 to point SCP AutoDiscoverURI to Exchange 2013 Client
Access FQDN by changing DNS entry for Autodiscover.domain.com to exchange 2013 CAS sever Ip
address

The internal DNS records should point to the internal host name and IP address of your Exchange
2013 Client Access server
Make sure that legacy.contoso.com resolves to CAS2007 in internal and external DNS.

Do you mean in the cross forest, the Exchange is using the OWA's External URL when it is looking for the destination OWA from the autodiscover? And to find the correct autodiscover XML I hope it is using the targetAddress attribute?

Do you know any method to troubleshooting this? So far I have not success to get any other error message than above. As I do have OWA's external and internal URLs popoluate.

And as I mentioned, Outlook is working normally, profiles are updated depends where the mailbox locate. And also the availability service is working for both direction. Only question is, how to get OWA redirection to work cross forest.