
Email messages that are sent will go to an SMPT server. The server will then look to see if an MX or Mail Exchange record exists. If the Mail Exchange is present the next step is to check and see if the username associated with the email is valid. This can be done from your computer without the need to send a test message.
This process will first require you to install Telnet for Windows. Click the Start button and then go into the Control Panel. Select the option for Programs and Features. You will then need to click on the entry to turn Windows Features on or off. Put a check in the box for Telnet Client and click the OK button to continue. Click the Start button and type CMD into the search box. Click the link to open the command prompt and type Telnet. If the program runs, it is installed correctly.
Type the command nslookup -type=mx gmail.com and press the Enter key. The command will seek out all the MX records for the domain and display them in an list. Repeat the command and use the domain of the email that want to verify. You will see a list of multiple MX records. Choose a record that has the lowest preference number to use for a test message.
Type telnet and the and the address of the record such as gmail -smtp-in.1.google.com 25 and the press the Enter key. Type a word such as hello to verify a connection to the other server. Type a command to identify yourself by using a fake email such as mail from:<example.example.com> and press the Enter key. Type in the recipients address using the rcpt to: command and press Enter.
You will be able verify if an email is valid by the response that you receive. If an email address is valid, an OK will appear on the screen. When an email address is not valid, a 550 message will appear. This message displayed will say that the specified email account does not exist or that the email account is disabled.

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