We found out a problem with Nokia Lumia 820 and Windows Phone Outlook 2010 not syncing emails but giving error 800C0000. After some debugging we found the following solution that is now added on on our email usage instructions page:
Problem: Nokia Lumia 820 Windows Phone Outlook 2010 says error code 800C0000 while synchronizing emails.
Solution: This problem is a bug in Windows Phone Outlook 2010 client caused by bad email content such as special characters. Other email clients can handle this situation, but Windows Phone unfortunately not. Typical problematic characters in emails might include for example:
The issue can be resolved by moving the offending email to some other sub-folder. Log in to webmail: https://mail.phz.fi then search for special characters not displaying properly. Select the message by clicking the check-box on the right. Use “Move” button to move the message to some other folder (such as Drafts).