The autoresponder module allow a site visitor to sign up for a series of emails sent each day via a cron job (uses Drupal Cron).
A site visitor wants to sign up for a N-day series of emails. In a block, he enters his email address, select
a preferable set of emails and clicks submit. Any emails in the system set to 'Day 0' are immediately sent. Each day thereafter, a cron job triggers the sending of emails appropriate for that day (the day user is on). If at any point the user wishes to unsubscribe, he can click a simple link at the bottom of the email to do so.