When I use the Bookmarklet or the Browserextension and click the button to send the current page to my kindle, it´s always ask me to enter the Email address.
My Browser delete the cookie on exit. One way to prevent this is to allow to pass the Email address as an url parameter to http://fivefilters.org/kindle-it/send.php
where domain=1 is e.g.: @kindle.com
Also in all Browserextensions there is a way to store this information.
providing this option will be very helpful!