Have a closer look at your campaign stats. If you notice that the open rate is taking a nosedive and the number of bounces goes sky high, it most likely means that your deliverability rate is at a rock bottom and your messages either end up in the SPAM folder or are not delivered to the recipients at all.
These symptoms may indicate that your domain reputation got seriously damaged or perhaps you even got blacklisted. Don’t worry, though, you can bring your good reputation back. Here’s a step-by-step manual on how to do it.