How can I send an email to a user supplied email address with an HTML form?

<form action="" name="myform" method="post" enctype="text/plain"
onSubmit="document.myform.action = 'mailto:' + document.myform.address.value">
Address: <input type="text" name="address"> <P>
Content: <textarea name="content"></textarea><P>
<input type="submit" value="Submit">
</form>