If you had members already sign up for a subscription with their email address information prior to integrating with one of our Marketing auto-responder options in MemberPress > Settings > Marketing tab, these existing user email addresses will not automatically sync and get added into your newly integrated auto-responder list. Only new users signing up, after you've integrated your preferred auto-responder, will be automatically added to your email list.
To add the existing email addresses of the members who did not get synched with your auto-responder, you would need to head over to MemberPress > Members and export a CSV file of all of your members. Filter out the names and email addresses that need to be added. Then you can use that information to import all the names and email addresses into your auto-responder.