Migrating a mailbox to Exchange 2013 in an mixed environment from Exchange 2010 can cause Outlook to be unable to connect to the mailbox.
This is caused because the autodiscover data is not updated automatically with the new location of the mailbox. CAS 2013 servers are still proxying the request to the 2010 mailbox servers.
By recycling the MsExchangeAutodiscoverAppPool on the CAS 2013 servers, we can force the autodiscover service to update the mailbox location data
Another option is configure intervals to be executed every X number of minutes (f.e. 30 minutes).
This is interesting when executing batch mailbox migrations during non-business hours. This way you ensure outlook clients will connect succesfully when the users start to access their mailboxes.