It is possible that one of the email providers was shut down which left their emails disconnected and that induced the spike in bounces from warmup.
Those are soft bounces which are not too damaging to your email reputation and there's nothing you need to do on your end. We monitor bounces in the warmup pool and then remove accounts from warmup if they cause too many bounces.
It might seem that there are more bounces than there actually are as your ESP will try to resend the email multiple times (about 3 to 6 times) within 72 hours causing the same bounce to be sent multiple times.
It’s the nature of warmup unfortunately. Bounces will never be zero, but we do our best to remove bad accounts fast.
