Bug introduced in patch from issue 3111185.

The empty index operator is not supported for strings anymore - ref
http://php.net/manual/en/migration71.incompatible.php

Removing all calls to mailsystem_set as this should be set be the user in the Mail System module settings.

Olof Bokedal authored Mar 10, 2015 and Hans Fredrik Nordhaug committed Mar 10, 2015

Issue #2397879 by olofjohansson: "Select all" checkbox should trigger the change event for effected checkboxes

The value can be modified in hook_mail_alter - ref API documentation for drupal_mail.

Issue #2225631 by hansfn: Using filter_fallback_format() instead of hard-coded value for message text format.