The WordPress Users List
MailPoet automatically creates a special list called WordPress Users. This list contains all of the existing WordPress user accounts on your website. This is especially useful if your run an eCommerce store or a membership-based website and want to send e-mail newsletters to your customers/members.
Note: the WordPress Users list does count towards the 2,000 subscribers limit of the free version of MailPoet. For example, if you have 1 "regular" subscriber and 2,000 WordPress users, you'll need to upgrade to Premium in order to send.
If you think the number of subscribers inside this list doesn't match the number of your existing WordPress user accounts, you can force a resynchronization by clicking on the Force Sync link when hovering over the list name: