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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.
Current version of GrayWall: http://www.ararat.cz/download/graywallsetup-1.1.0.exe
Documentation: http://www.ararat.cz/download/graywall.pdf