When email is your main communication tool it's easy to get confused and forget to respond to the client. In Gmail you can easily add a button that indicates about the need to respond to the email.
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Email Reply Status is an extension for Google Chrome that adds Replied/Not replied button to email.
After installing the extension, all emails will automatically start to display the statuses: Replied and Unanswered. So you can understand to what an email you should respond.
The status can easily be changed using the drop-down menu.
Install Email Reply Status