To connect your HTML unsubscribe form to Direct Mail:

The <form>
opening HTML tag should look like the following: <form method="post" action="http://ethreemail.com/e3ds/u.php">

The body of your form must contain the following two inputs:
<input type="hidden" name="group_e3ds_id" value="the ID of your subscription group">
<input type="text" name="subscriber_email">
group_e3ds_id
is the ID of your subscription group. You can find this ID by double-clicking your subscription group and examining the default subscribe URL. The portion of the URL following "g=" is the subscription group ID. For example, if the subscribe URL displayed by Direct Mail was http://ethreemail.com/e3ds/u.php?g=a12345, then group ID would be a12345.
subscriber_email
is the email address of the subscriber.

The body of your form can optionally contain the following inputs:
<input type="hidden" name="unsubscribed_url" value="the URL of your custom unsubscribed page">
unsubscribed_url
is the URL of your custom unsubscribed page. This page should inform the user that they have been removed from the mailing list. This URL will be passed two query string variables: e=(email address of the unsubscribed user)
and g=(the subscription group ID)
.