FlexMailer is an email delivery system for CiviCRM. It does not define any user interface; rather, it replaces some internal parts of CiviMail and provides new APIs. Other extensions can use these APIs to provide more fine-grained control over email delivery.
A few example scenarios supported by FlexMailer:
Composition: Change how email messages are composed. CiviMail has built-in support for simple tokens. With FlexMailer, an extension can change this in deep ways, e.g.
- Compose messages with Twig, Mustache, or some other language.
- Improve the performance of Smarty-based mailings (by disabling CiviMail tokens).
- Delivery: Instead of delivering email messages over SMTP one-at-a-time, deliver them using batched SMTP or batched web-service calls.
- Tracking: Change the tracking codes used for click-throughs or opens.
For more detailed discussion, see https://github.com/civicrm/org.civicrm.flexmailer/.