Flexailer is distributed as part of CiviCRM "core" since CiviCRM 5.28. Therefore it is not required to install it separately. If you are upgrading from an older version, you can delete the copy of the extension in your "ext" directory (wp-content/uploads/civicrm/ext/ on WordPress, files/civicrm/ext on Drupal), then go to Administer > System Settings > Extensions, and click the Refresh button).
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.