Presumably you're trying to send styled email and the text box will contain the text of the mail. I suggest that you do two things -- first use a DIV to display the text to the user. You can style it so that it looks like a text box/area if you want with borders, etc. Then have a hidden input that actually contains the data that will be sent back to the server. This way your display works the way you want, but the data is passed back intact for the email to be sent. Alternatively you could let the server format the email and pass the data back as JSON or some other format that can be interpreted by the server.
var sendEmailText = "Hi, you found these items<ul><li>Car1</li><li>car2</li></ul><br><br/>Bye!"
jQuery('#receiverMessage').val(sendEmailText);
jQuery('.receiverDisplay').html(sendEmailText);
<div style="border: 1px solid #0000ff;" class="receiverDisplay">
</div>
<input type="hidden" id="receiverMessage" name="recevierMessage" />
Edit: If the user needs to edit the message, as indicated by your comments to another answer, then consider using a markdown editor such as WMD (used on SO).