Declined notices won't delete from inbox

I have a user who has received a “Declined” meeting notification in their inbox. The user cannot delete this document for some strange reason. I have tried compacting the database, running fixup on it and running a view repair CTRL/SHFT/F9 to no avail. I also tried opening the users mail file from the server as Administrator and deleting it to no avail. Does anyone know what causes this and how to fix it?

PS

The user has a local replica of their mail file which does NOT contain the document. I tried replicating without any luck.

Thanks in advance