During a recent rich co-exist deploymnet between on-premise Exchange 2010 and Office 365, I noticed therer is a problem establishing organizational relationship from Office 365 to on-premise Exchange coexist server when on-premise Autodiscover is published by a TMG server. Exchange 2010 federation uses SAML tokens—not user accounts—to authenticate against IIS for EWS calls. TMG doesn’t know how to validate SAML tokens, so the incoming requests can’t be authenticated and passed on to the Exchange Server 2010 coexist server. Every time a cloud user tries to view free/busy information of an on-premise user, there is a log in the TMG server indicating "Access Denied"
The end result is that we couldn’t get a proper sharing relationship setup and you can’t federate calendar data.
When we knew what the problem was, fixing it involved modifying the OWA and ECP virtual directories on all of our Exchange Server 2010 CAS servers to perform FBA, then modifying the Web listener on our TMG Server to disable pre-authentication. Finally, we needed to modify the authentication settings for each of the TMG publishing rules for ActiveSync, Outlook Anywhere, and OWA to set them to No Delegation, But Client May Authenticate Directly, and to revise the Users settings from All Authenticated Users to All Users. Revising the Users settings is important; without this, ISA won’t pass any connections on to the CA servers. You may also need to verify that the authentication settings of your other Exchange virtual directories are valid; many organizations will allow basic authentication between TMG and their CAS servers, but require NTLM or Windows Integrated from external clients to TMG.
If you’re using multiple Web listeners to publish Exchange 2010, then your step may be different. The federated delegation feature requires direct access to the EWS vdir on your CAS servers, and EWS is typically published on TMG as part of the Outlook Anywhere rules. If you publish those using a separate Web listener—which will require a separate IP address, FQDN, and SSL certificate—you can simply disable pre-authentication for that Web listener, but still allow it on your OWA, ECP, and EAS directories.
Calendar sharing and TMG FBA pre-authentication are both wonderful features, but at this point they are not compatible.
I'm publishing Exchange 2013 from behind TMG. I get an error with Outlook 2010 users when trying to share their calendar. They get a message stating that there was an error creating the sharing request, however Outlook 2013 works fine. Is the reason for this as per your article? I can find a TechNet article for Outlook 2007 not working, but there doesn't appear to any formal recognition of the same for 2010.
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