Read only access to users to their OWN mailbox.

HI , we have EX 2010 environment and i am trying to give read only access to user to their OWN mailbox , so they can not send or delete any mails from their mailbox .

This is Client requirement . i used Add-mailboxpermssion -identity username -user username -accessrights readpermission .

But is does not work , user sill able to delete mails from their mailbox ,

And if i remove "NT AUTHORITY\SELF " from mailbox permission it does not allow user to logged in to OWA .

Any suggestions on this,