Do you have GrayWall?  This should help.

Files: GrayWall 1.0 Documentation

"Graywall is a program that adds a graylist (or greylist) feature to the Mercury/32 SMTP server. It uses the Mercury/32 API facility that has been introduced in version 4.51. You must have at least this version of Mercury/32 to run Graywall!"

http://community.pmail.com/files/folders/mercadd/entry3505.aspx

Mercury for Windows

Is there a way to set a limit to the number of emails from the same email address within a given time period? If the number of emails from the same IP address exceeds, say, 10 in 10 minutes, could we block the IP address for the next 3 hours or so...