I published a solution to this using extend.php
in the past, unfortunately the solution will no longer work in Flarum 1.x https://discuss.flarum.org/d/24887-subject-and-sender-in-notifications/4
But the logic is still the same. You would have to create a service provider and call
$this->container->afterResolving('mailer', function ($mailer) {
$settings = $this->container->make(\Flarum\Settings\SettingsRepositoryInterface::class);
$mailer->alwaysFrom($settings->get('mail_from'), $settings->get('forum_title'));
});
And then register that service provider in the root extend.php
with the Extend\ServiceProvider
extender.