I recently migrated all of my mailboxes to Exchange 2013 from 2010. I've noticed that if I try to connect with an Outlook 2010 client, the first two steps of auto discover able to complete but on the third (log on to the mailbox) I received a message that the server is unavailable. If I attempt to manually create a profile and type in the name of the Exchange 2013 server instead of using auto discover, I get a similar result.
Interestingly, I have no issues connecting with Outlook 2013 or Outlook 2007 or OWA.
This happens on multiple machines and only occurs with Outlook 2010. I've tried enabling and disabling encryption in the Outlook profile and changing the authentication method used to connect to exchange and none of those things seem to work.
I've also confirmed that there is no mapi version blocking set on either the client access server or the mailbox level.
Thoughts welcome.