User preferences: 'Someone replies to one of my posts (email)' and 'Someone mentions me in a post (email)' are turned on by default for each new user in your Flarum forum.
composer require piotr-tokarczyk/flarum-user-default-preferences
composer update piotr-tokarczyk/flarum-user-default-preferences
php flarum cache:clear
Enable the extension.