You can try to re-index the users by login as the domain admin and go to Settings | Domain Settings | Users and then select the users and click on the reindex button.  This will cause the users to be re-index and based on the about of emails they have it will take some time.  If after waiting they are still stuck in a index status you can manual delete the index folder off the server.  To do this stop the SmarterMail service and go to the users folder (default path C:\SmarterMail\Domains\(domina name)\Users\(users name)) and delete the Index folder.  Then start the SmarterMail service and do another re-index.  If you follow these steps it should resolve the issue.  If it does not you will need to open up a support ticket.

Also I see according to your post you are running version 11.  In version 12.5 there is a fix for account that get stuck in a index loop.  If your license allows you to upgrade to this or later version this should also resolve the issue.