askvortsov or it should be listed here https://docs.flarum.org/mail.html
It's just like the (now removed?) Amazon SES driver, which required installing the Amazon SDK when in use.
I don't think it would be particularly bad to include Guzzle as a Flarum core dependency since so many extension use it already, but technically it's only needed with this particular driver so it shouldn't be needed all of the time.
Either should work. It's the suggested dependency for illuminate/mail, and we can see they say either 6 or 7 can be used https://packagist.org/packages/illuminate/mail (see under suggests)
