Provider has enabled option "Accept emails from authorized users only" in the properties of Trouble Ticket system in BA CP:
Home - System - Settings - Helpdesk
However when unautorized user send e-mail to the mail collector, e-mail is accepted and BA does not send rejection message.
The behavior is by design - e-mail is accepted anyway, but the option only sets if the ticket will be created or not.
Internal issue PBA-23254 has been submitted to rename the Helpdesk setting to reflect its real functionality. Internal issue PBA-67390 has been submitted to implement email rejection notifications.