I am using Outlook 365, and I cannot delete all the messages from my Junk Mail folder. I always get an error "interface has returned an unknown error.  If the problem persists, restart Outlook." even after restarting several times. 

Also, "Empty Folder" does not work. 

What can I do to cleanup that folder?

Hello,

Might be you are connect with Outlook 365 thats why you are facing this problem. Actually, when you work online the junk mail filter list are saved on the server. In that case, you are not able to clean up your junk email folder. So you have to contact to your OWA administrator.

For more information: https://support.office.com/en-ie/article/Overview-of-the-Junk-Email-Filter-5ae3ea8e-cf41-4fa0-b02a-3b96e21de089

Note: Improve community discussions by marking the answers helpful.

Thanks & Regards

Clark Kent

Hi,

The error message implies there might be something wrong with your Outlook data file(.pst/.ost), please run Scanpst.exe to scan and repair the issues in the data file:

https://support.microsoft.com/en-us/kb/272227

Regards,

Melon Chen
TechNet Community Su