An increasingly common way for senders to send spam is to use the recipient's domain name, or e-mail address as the FROM address in the sent e-mail. Since the sender is a local domain account this will bypass most anti-spam filtering techniques.

If you are receiving e-mails that are bypassing the Anti-spam engine, as shown in the anti-spam logs, there is way to stop this behavior from happening.

Open up the Icewarp Console and go to the [Mail service] [Security] [General Tab] and check the option:

'reject if originators domain is local and not authorized'.

Once this option is checked, only accounts that have authenticated to the server in some fashion will be able to send to your server using a local domain address. Authentication must happen in one of the following ways: POP before SMTP, Trusted IP/Host, or SMTP Authentication. These forms of authentication will allow legitimate users to still use the system properly.