Working on an Exchange 2003 to 2010 transition I came across the “unable to open default mail folders. You do not have permission to log on” error.
After a bit of troubleshooting, including; verifying OWA access, Autodiscover, isolating the issue to all mailboxes whether migrated or newly created 2010 mailboxes and recreating the Outlook profile, the issue came down to DNS.
The customer environment was using a fqdn of subdomain.domain.org and an A record for the CAS array name was never placed into the subdomain zone. Once added the issue was resolved immediately.