To enable SmartSerialMail to automatically collect subscriptions created by the Opt-In form and to add their subscriber information to the recipients list, you have to configure the returns options in SmartSerialMail as follows:
At first you have to set the field "Handle as subscription if subject contains:" auf "Subscribe me" or to the subject you may have set in the "default_settings.php" file (See section Advanced Settings for more information).
Now enter enter the e-mail address from which your web server sends the subscription mails to the "Trusted Subscriptions From Address:"-field:
At last choose the list in which you want to collect the subscriptions. Now SmartSerialMail should be able to collect all subscriptions that were send by the Opt-In form.
For more information about the collection of subscriptions, please read the chapter Subscriptions.