Right now, everyone who uses sign in with Google is automatically marked as verified. While all email addresses are validated by Google, not all of them we can actually send to.
Because of our target audience (largely 14-17 year olds) we have a lot of people using their school Google accounts to sign in, which are largely unable to receive any external mail. At the same time we have many people who use their personal accounts to sign in so disabling Google sign-in is not an option.
I propose a new setting to require all Google users with non-@gmail.com or @googlemail.com email addresses to verify their email separately at signup (and possibly a setting for a list of domains where this won’t apply).