Newsletters: Import Subscribers« Import/Export
The WordPress mailing list plugin allows you to import subscribers into your database from either a CSV spreadsheet or a Mac OS address book vCard file exported from your Mac OS operating system.
All subscribers imported will be assigned to a specified mailing list that you choose and will be activated immediately in order to start receiving newsletters. If the chosen mailing list is a paid list, the subscribers will still be activated and their subscription will expire after the specified subscription interval for that list.
Imported subscribers are exactly the same as normal subscribers, so they can later on subscribe to additional mailing lists, unsubscribe from mailing lists, renew subscriptions to paid lists and all the other features available to normal subscribers.
When subscribers are imported, each record is being checked for duplicates already in the system, invalid email patterns, users in the WP database registered with that email address and much more to ensure that the system is stable and acts as expected.