I have 3 MBX servers, one server has an issue with web services so to do some testing I mount a mailbox database on the affected server with a new test mailbox test.user@domain.com on affected server SERVER3

if I run the cmdlet test-outlookwebservices -identity:test.user@domain.com -mailboxcredential (getcredential) is see the source as SERVER3 with autodiscover test FAILED as expected

now if I run the command again the source has changed to mailbox server SERVER2 (which works) and the Availibility Service FAILS autodiscover and EWS works

Why/How does it change is the user mailbox/ mailbox database that test.user@domain.com is in is mounted on SE

Hi,

According to your description, I understand that you get different source when run same command to test outlook web service.
If I misunderstand your concern, please do not hesitate to let me know.

I suspect that the server which are you used to run this command is different. Basis on my test, when I run this command in Exch5-cu1 (mailbox server), the source is Exch5-cu1.cu1.com and vice versa.

For your question, it indicate that Server3 cannot connect to autodiscover services. Server2 can connect to autodiscover services, however cannot return the free/busy information from CAS server. Running the cmdlet on a Client Access server will test the local server using the test mailbox user created earlier, you can also perform the test for a specific mailbox by using the Identity and MailboxCredential parameters.
Note: Testing a specific mailbox is useful if you are troubleshooting problems with one or more of the Outlook Web Services in a particular site within your organization. You can compare results between test mailboxes in different sites to help you narrow down the source of any problems youre seeing.
Additional, please refer to below link to get more details about Troublshooting Autodiscover:
http://blogs.technet.com/b/exchdxb/archive/2012/05/10/troublshooting-autodiscover-exchange-2007-2010.aspx

Best Regards,
Alle