EfekanKksal I believe you. But try to understand - by typing there anything different that string "smtp", "mailgun", or other supported name of driver, you will get the same. You could write characters, but wrong ones, like "aaaaa", and you will still get white page. It is not about numbers. By writing basic config wrong, you break your forum.
I can only agree with one thing - now you probably have to change this config directly in database, and have no option to change it in Flarum GUI. There should be some mechanism to change it, and some other to warn you, that you are doing it wrong. But core devs are already working on that: flarum/core1169
