Handy Geek Links with instuctions

Troubleshooting DNS:

kloth.net

Troubleshooting the misbehaving website:

rexswain.com http viewer

Troubleshoot MX records:

mxtoolbox.com

Track down the random ISP .txt records or the exchange server that will not work remotely….

Microsoft Remote Connectivity Analyzer

https://testconnectivity.microsoft.com

IF all else fails … did you check the hosts file??