On our cloud platform, we handle email for you. To ensure email deliverability we have to keep our "score" high which means we are very strict about managing email sending. Right now, our systems monitor when an email bounces or is reported as spam. For spam reports we immediately stop sending to that email to ensure a good sending score. For bounces, it depends on the type of bounce but they will eventually be blocked as well. This is great for sending scores but sometimes emails get