I have this situation:
Exchange Server is 2010; Outlook 2007 on Windows-7:
UserA email account gives “messaging interface returned unknown error. Restart Outlook” when I select "digital signature and/or encryption" with a good user certificate, when Sending an email.
This error definitely is related to profile/data-file since it's localized to this user. Recreating profile does not fix it. I deleted the data file outlook.ost and restart outlook, new outlook.ost was created. Problem still remains.
Other user accounts can send email signed/encrypted. But UserA cannot sign/encrypt. It will send plain email without email security. Several online suggestions have not solved the problem yet. Any other suggestion will be appreciated.