TCP/IP-channel-specific options: BANNER_PURGE_DELAY (integer)
(New in MS 6.3.) 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 start blasting SMTP commands as soon as the connection is open. 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.
BANNER_PURGE_DELAY SMTP server option exists to implement this strategy. Set the
BANNER_PURGE_DELAY option to the number of centiseconds to delay before purging the input buffer and sending the banner. The default value of 0 disables both the delay and purge. See also the PORT_ACCESS mapping table
$D flag, which can also be used to set such a banner purge delay, in a more selective manner And note that the
disconnectbadcommandlimit channel option may be of interest when using any such banner purge delay technique.