Format filter has no effect

Problem/Motivation

The client of the Drupal 10 site I am the administrator for requires to send a notification email to the user when the user account is created. Under admin/config/system/mailsystem I set the Default Mailsystem to Mailgun mailer for both the Formatter and the Sender. In the Mailgun module advanced settings I set the format filter field to Full HTML. The recipient still gets the email as plain text.

I previously had issues with sending emails to users with Gmail accounts as Gmail seems to have been blocking them. I am thus using Mailgun to ensure the users with Gmail accounts receive the emails.

I first attempted to combine formatting as Mime Mail mailer and Sender as Mailgun mailer, but ended up with the active issue https://www.drupal.org/project/mailgun/issues/2684451 → . I have to add that the last patch added to this issue does not work.

Also, in the Mailgun module documentation → it is stated that Mime Mail or any other HTML Mail module is not necessary, as the Mailgun module can carry out of the HTML formatting. Please note that the link under "You can read more about Mailgun dependencies here." produce a 404 error.

Steps to reproduce

• Drupal core 10.3.1
• Mailgun 2.0.0
• Set the Full HTML and Basic HTML on /admin/config/content/formats to CKEditor 5.
• In /admin/config/services/mailgun/settings advanced settings set the format filter field to Full HTML.