Issue

My Customer introduced an Exchange 2013 server into the Exchange 2010 based organisation, however found that Inbound Free/Busy tests failed using Office 365 based OWA.

The W3SVC IIS logs revealed the following error – the line has been shortened for clarity

POST /autodiscover/autodiscover.svc/WSSecurity & ASAutoDiscover/CrossForest/EmailDomain//15.01.0225.018 - 500 0 0 124

Cause

The Exchange 2013 server was unable to resolve mail.domain.com and autodiscover.domain.com to itself, as internal DNS had not been updated yet. This was resolved by adding static  entries for these names into the HOSTS file.

After re-testing, the issue was found to have been resolved.