These are the online networking tools that would be useful for troubleshooting network problems:
http://www.all-nettools.com/tools1.htm;http://www.samspade.org/;http://us.mirror.menandmice.com/cgi-bin/DoDig;http://www.checkdns.org/;http://www.simplelogic.com/net_utils/;http://www.blackcode.com/net-tools/;http://www.network-tools.com/;http://www.antionline.com/tools-and-toys/
And to check for open relay mail servers, you can check for these sites;
http://www.ordb.org/;http://www.mail-abuse.org;http://www.dsbl.org
I'll just update my blogger for other tools ;