Firstly, to be able to use Autoresponders you will need to have CRON or Scheduled Tasks set up on your Server. For instructions on how to do this please read these knowledge base entries on setting up CRON and setting up Scheduled Tasks.
Once this is set up you will have to tell Interspire Email Marketer that you wish to use this feature by selecting 'Cron Support' from the settings page of Interspire Email Marketer.
This will display a warning that will disappear once Interspire Email Marketer detects a successful CRON/Scheduled Task running.
Now you can use Autoresponders. Once you have created an Autoresponder Interspire Email Marketer will use the cron.php file, when it runs, to check what contacts match the criteria for the Autoresponder (time after signing up in particular). Interspire Email Marketer will then send to all the users that match this criteria.
This will happen every time the cron.php file is run.
