If you use a ND6 admin client to get a console on R5 server don’t type „tell router update config“. Somehow this is interpreted as „tell router use config“ and shared mail will be enabled on this server (config.nsf or mailobj.nsf will be created and Shared_Mail is set to 2). You’ll have some fun with mail attachments!
Stop the router task, unlink the object database, delete the object database, set Shared_Mail=0, restart the router task.