I have only used Formail a few times, but hopefully this will shed some light,
similar to what oeb said, you need to label the fields
Something along the lines to
Code:
<label id="Label1">doing</label><input name="doing" type="text" />
<label id="Label2">email</label><input name="email" type="text" />
then
Code:
<input type="hidden" name="required" value="doing,Email" />
Also you may know already, but, it will look at look better if you include your own error page, so if the fields required are not filled in, it will divert to your error page.
use this
Code:
<input type="hidden" name="missing_fields_redirect" value="url to error page" />
Hope this helps