I cannot really help.
I just tried to install in a brand new installation of Flarum and composer says :
Using version ^0.1.0@beta
for vingle/flarum-configure-smtp
And the version is the old one.
I uninstalled and reinstalled from dev-master and then I get my first version, with the Mail Driver field, but not the dropdown list that @santiagobiali added. And in this situation the extension is dangerous : if you put an unsupported value, flarum no longer starts.
Perhaps something to adapt to version beta-5.
@santiagobiali can you help, please ?