Sorry if this is in the wrong place.

Trying to figure out how if at all possible to do this.

We have a shared exchange server with multiple tenants.  And configured to use adfs for sign-on which is working.

However we now had a situation where customerC wants to have his mailboxes on our servers.  But using his adfs to authenticate for access to them

There isn't a trust setup between the domains either.

I've managed to get our adfs server to send the request over they customers-adfs for authentication.  But once i've been authenticated I get the following error on the owa page

owa/auth/errorfe.aspx?msg=UpnClaimMissing

X-ClientId: VAIG - DBJW - KKED - NZYYEWQ
X-FEServer: TEST08
Date: 5/8/2015 6:05:12 PM

I recommend that you post this in the Active Directory Federation Services Forum: https://social.msdn.microsoft.com/Forums/vstudio/en-US/home?forum=Geneva

Hi,

We may need to create relying party trusts, heres a blog about Configuring Exchange 2013 SP1 to Accept AD FS Claims, for your reference:
http://blogs.technet.com/b/platformspfe/archive/2014/03/12/configuring-exchange-2013-sp1-to-accept-ad-fs-claims.aspx

Since this issue is related to Active Directory Federation Services, please refer to above link as Ed mentioned so that you can get more professional suggestion about your question.

Thanks