When I click on the Queues node in ESM, in the right-hand pane I see tens of
listings which read: "Internet Mail (SMTP) Connector (<server_name>) - <some strange websites> (SMTP Connector)"
The in the Message Tracking Center I see hundreds of Delivery Status Notification (Failure) messages.
So I assume that this is spam: all those websites are trying to send me messages that are not delivered, so my postmaster is notifying them of NDR.
How can I stop them from even hitting my server?