This page contains links to various third-party services that we've found over the years for testing various Internet facing services for our clients. Some of these also provide valuable troubleshooting details that are useful for identifying the underlying cause for an issue that you are seeing.
Network Tools is our go-to general purpose test site for troubleshooting Internet connectivity and DNS. We also use it for back tracing IP addresses to their owners.
For general purpose SMTP troubleshooting, we recommend MXToolbox.com. This will check your SMTP servers to ensure that they are running and show the conversation that occurs to assist with troubleshooting.
Wormly is useful for testing against specific SMTP hosts. This is useful for ensuring that a specific server is operational. Other mail tests will randomly select a server or only obey the MX record priorities that you have specified.
For DNS health analysis, we recommend IntoDNS.com. This will query the root servers for your nameservers and verify that they are online and configured similarly. It will also perform a check for MX records (and common misconfigurations) as well as a default WWW record.
There is also DNSStuff.com. This is a tool that is turning into a paid service, but it still has many free tools.
For DNSSEC, give the Verisign DNSSEC Debugger a try.
The big mac-daddy of tests for Microsoft services exposed to the Internet. Tests Exchange, Skype for Business (Lync/OCS), and Office 365. Also has a very useful email message header analyzer that makes it simple to read headers and figure out transfer delays.
This is a very nice, web-based FTP client that supports active and passive modes as well as SFTP. Dead simple to test your FTP server's access from outside.
For looking at page load times and waterfalls as well as headers and other attributes for a site (not to mention views over various bandwidth links and browser types), this is a good tool.
Another service for checking the performance and suitability of a site for a given platform. Fewer controls than Web Page Test, but it runs faster and highlights areas of concern.