Problem
You need to use telnet from a command prompt. However, this feature is not installed by default.
tl;dr
From PowerShell or Command Prompt:
dism /online /Enable-Feature /FeatureName:TelnetClient
Solution
Windows versions starting with Windows Vista do not come with Telnet client installed, therefore you will need to manually add it if you suddenly find a need to verify a mail server, or unsecurely connect to another machine. There are two ways to install Telnet on Windows. The first one is the typical Windows way - navigate a dozen menus, place a checkmark and click OK. The second one is a command you can enter in Command Prompt or PowerShell.
Install via GUI
1. Open Control Panel.
2. Click on Programs and Features.
3. Click on Turn Windows Features on or off.
4. Place a checkmark next to Telnet Client, and click OK.
Install via Command Prompt or PowerShell
This should work with any version of Windows starting with Vista:
dism /online /Enable-Feature /FeatureName:TelnetClient
If you are using PowerShell, you can also run this:
pkgmgr /iu:"TelnetClient"
Whichever way you installed it, you can now use Telnet, either by opening a separate client, or by using telnet 11.22.33.44 from the Command Prompt, where 11.22.33.44 is the IP address you wish to connect to.