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!
Information about graylisting in general can be found at: http://en.wikipedia.org/wiki/Greylist. We recommend that you read this first.