The simplest way is to use <a href="mailto:
[email protected]">quick reply</a>
This only works if a mail client is configured on the browsing computer.
If you're talking about a text box where people type in a comment and submit then it gets a little trickier.
You have to create a form that submits to a script that creates and sends an email. I can do this in ASP but I don't know what server environment you're working in. The actual textbox for the comment is just <input type="text"> or <textarea></textarea> depending on how much of a message you want them to send. But without a script to build and send the email the reply box won't work.