I used Google AppEngine for a mobile game that sent email to notify players. While there was a link to turn off emails in every email and a setting in the app,

I used Google AppEngine for a mobile game that sent email to notify players. While there was a link to turn off emails in every email and a setting in the app, enough people clicked "mark as spam" that we got shut off. It happened a couple of times and each time Google turned us back on. My general experience is that Google is more heavy handed than a true ESP like Sendgrid or Postmark. ESPs take spam seriously, but they also know spam reports are a fact of life.