Over the past 24 hours, I've had GitHub request that I re-verify my gmail three times (roughly 22 hours ago, 2 hours ago, and now), each time resetting my primary email's status to "Undeliverable" and "Unverified" The triggering event may be an email bounce. I get a lot of github notifications sent to my email, and the failure of just one/a few may trigger the reverification.
When this happens, you can spin up a temporary server and have a mechanism in place to redirect email so you don't go down when your provider does.