On December 14th through December 15th, Gmail experienced ongoing outages throughout the day. Here's a breakdown of what happened and how to handle it:
Here's what we know so far: Some email addresses at Gmail were undeliverable during the outages throughout the day. So this may have resulted in a higher number of bounces on your emails that you sent at any time on the 15th or in the days after. This means that anyone that bounced was placed on our global block list.
How to know if you were affected: Check your email stats from emails sent during this period of time and compare it to your usual bounce rate for an audience of a similar size.
We recommend also pulling a report of anyone who has a subscription status of "bouncing" or "previously bounced", and targeting the subscription date (which is really subscription update time) to the last 24/48/whatever hours (from whenever you're reading this article). That report will give you your "normal" bounces, but also anyone that was affected by the outage. Keep this handy, you will need it later.
Note that you won't only see Gmail users on the report. Many organizations use Gsuite, which gives users their own email domain. And some number of bounces are normal, so you'll see other email providers as well.
Here's how to resubscribe your bounced activists: We've removed the bouncing activists from the global block list. Use the report you pulled (see above) to resubscribe these folks to your list. Use the buttons on the right side of your report manage page to resubscribe folks on that report to your list.