following yesterday's short exchange with @datitisev I have managed to get my extension into working order. So, here it goes:
Mail Filter extension for Flarum - filter e-mail registering by your sites with whitelists, blacklists and regular expression describing both.
This extension was built and tested on Flarum 0.1.0-beta12 and PHP7.4. If you are having issues and a different build, please mention that you didn't use the same setup as this (so I know to test it on another environment).
Other than that, there are no prerequisites. It's really a minimal extension.
Use Bazaar or install manually with composer:
composer require studosi/mail-filter
composer update studosi/mail-filter
More information on GitHub!