There could be many reasons why emails are being bounced back on Seller Central, the most common ones would be if your buyer's inbox is full or if they signed up to Amazon with a non-valid email address for example.

When this happens, feedBAKER would still send messages to these buyers but these will bounce back and you will receive notifications about it in your Seller Central inbox:

What we normally advise is for you to blacklist these buyers, so that no further emails will be sent by feedBAKER and you won't receive undelivery notifications from Amazon.