14.10.1 Anti-Spam Technique: Delay Sending the SMTP Banner

A useful spam-fighting strategy is to delay sending the SMTP banner for a brief time (half a second, say), then clear the input buffer, and finally send the banner. The reason this works is that many spam clients are not standards-compliant and starts spewing SMTP commands as soon as the connection is open, ignoring any responses the server sends. Spam clients that do this when this capability is enabled will lose the first few commands in the SMTP dialogue, rendering the remainder of the dialogue invalid.

This feature has now been implemented in Messaging Server. It can be enabled unconditionally by setting the BANNER_PURGE_DELAY SMTP channel option to the number of centiseconds to delay before purging and sending the banner. A value of 0 disabled both the delay and purge.

The PORT_ACCESS mapping can also be used to control this capability. Specifying $D in the template causes an additional argument to be read from the template result after the mandatory SMTP auth rulset and realm, and optional application information addition. This value must be an integer with the same semantics as the BANNER_PURGE_DELAY value. Note that any PORT_ACCESS mapping setting overrides the BANNER_PURGE_DELAY SMTP channel option.