This is one of the scariest buttons I've ever seen: When would I ever want to email everybody in my Gmail contacts list? everybody??
From my experience regular users sometimes prefer sharing with everyone instead of taking the time to select specific contacts. They don't really care, it's just email...
I saw the email from a contact, clicked through and saw that your app only asks for "view contacts" so I though I'd be able to choose to whom I'd send an email or at least have some kind of confirmation before all of a sudden "BAM" you send thousands of emails to everyone I've emailed in the past 5 years.
That's really bad. You have to improve the UX/UI so that there's a bit of granularity and more transparency otherwise, you're going to lose your Google API access... or simply disappear as everyone marks your emails as spam.