Microsoft Exchange Server SMTPDiag Tool

http://www.microsoft.com/downloads/details.aspx?familyid=BC1881C7-925D-4A29-BD42-71E8563C80A9&displaylang=en

Troubleshoot network problems with PingPlotter

http://whitepapers.zdnet.com/download.aspx?&scname=Network+Administration&docid=288146

Troubleshooting TCP/IP Using Your Toolbox

http://whitepapers.zdnet.com/whitepaper.aspx?&docid=174249&promo=100511