If you happen to be using an external mail filtering service or appliance, one of the critical setup steps is to ensure that MDaemon is configured to not accept messages that attempt to bypass your mail filtering service as spammers look ways to bypass filtering gateways.
There are multiple ways to accomplish this in MDaemon, but one of the easiest ones is often overlooked: IP Shield. IP Shield is a very simple feature, it provides an administrator a simple way to tell MDaemon to only accept mail from a particular domain if it matches one of the listed IP addresses. Once upon a time, this was used to prevent spammers and others from forging one’s own domain, but there are better ways to accomplish this in MDaemon now, so today, we’ll use IP Shielding in another way: By using wildcards. With a wildcarded sender domain, you can use IP Shield to ensure that MDaemon will only accept mail if it’s from a pre-defined IP address or uses authentication.
Open the Security menu
Click on Security Settings
Under Sender Authentication, open the IP Shield dialog
Uncheck Do not apply IP Shield to messages sent to valid local users
Check Do not apply IP Shield to authenticated sessions
Check Do not apply IP Shield to Trusted IPs
Check IP Shield honours aliases
Uncheck Check FROM header against IP Shield
In the Domain field, enter *
In the IP field, the IP address of your mail filtering gateway
Click Add
Repeat these steps to add any other IPs that should be allowed to send mail without authentication.
Note that you can use wildcards and CIDR notation for IP addresses here.
Since users should be configured to use authentication, this will not impact normal user traffic, but it will block any unauthenticated attempt to send mail unless the IP matches one of the entries.