Was working on some modifications for the Print module's send to a friend functionality. In this case we have a legacy database that has several years worth of site registrations. If a user isn't registered for the site yet we want to send them an additional paragraph explaining how to get the registration code for the site.
By doing a
hook_form_alter() on the send as email form it's possible to extract the submit function for the form and have our own processor handle it. Here's a way to handle splitting the members and non-members up for mailing.