Troubleshooting may require both hardware and software tools.   You may not think of yourself as a hardware person, but a small kit of hardware tools is useful to have on hand.   A simple cable tester and a LAN maintenance toolkit (with every hand tool you'll ever need) can be bought at most places that sell LAN equipment.  

Add to those tools a dual-boot laptop, and you'll have all of the troubleshooting hardware you'll need.

Linux includes a large array of simple software tools that can help you isolate a problem through testing.   These software tools are sufficient to address most enterprise network problems.

SendMail testing

After the DNS zone has been created and a sendmail configuration has been built with the correct dnsbl (DNS-based black-hole list) feature, the new configuration can be tested using the -bt option with the sendmail command.   see the sample below:

  $ sendmail -bt -Ctest-dnsbl.cf

  ADDRESS TEST MODE (ruleset 3 NOT automatically invoked)

  Enter

  < .D{client_addr} 172.16.12.1

  < Basic_check_relay <>

  rewrite: ruleset 192 input: < >

  rewrite: ruleset 192 returns: OK

  < .D{client_addr} 192.168.72.37