What means x mailer drupal

A curious question!

X-Mailer is a header field in an email message that identifies the email client or server that sent the message. It's a way for the sender to declare the software or system that was used to send the email.

In the context of Drupal, X-Mailer is often used to specify the Drupal version and the PHP version that was used to send the email. This information can be useful for debugging and troubleshooting purposes.

When you send an email from a Drupal site, the X-Mailer header field is typically set to something like:

X-Mailer: Drupal/Courier (http://drupal.org) PHP/7.4.21

This tells the recipient that the email was sent using the Drupal CMS, specifically the Courier email module, and that the PHP version used was 7.4.21.

The X-Mailer header field is not required, but it can be useful for tracking and debugging purposes. Some email clients and servers may also use this information to filter or prioritize emails based on the sender's software and system configuration.