All of the "Front Door Rejectors" should only be used as a last resort. These filters reject incoming emails right away and therefore, it will be very difficult to troubleshoot if something goes wrong. We have had instances in the past where folks call our support department because they added someone either incorrectly or forgot about it and now there is no trace of the connection.

If you're getting hit by someone trying to guess passwords, make sure you disable SMTP Authentication for the primary port (25) and check Block Malicious IP Addresses under Server Configuration/Advance tab. This will automatically block IP addresses temporarily.