By default, all users in Rex will have email bounce notifications turned on. This notification ensures that you'll receive a notification via email to help you identify the email address that bounced, the affected Contact and the reason that was provided to us by the recipient's server.
While there is information in these notifications as to why your email has bounced, it may not be clear. Our response will only be as good as the response we get back from the recipient's mail server. For more detailed information on the codes, check out; https://sendgrid.com/docs/Delivery_Metrics/email_error_messages.html
Once you recieve a notification, you can then go into the Contact record and fix the bounce. For information on how to do this, check out: Invalid Emails in the Contact Module
For information on enabling / disabling email bounce notifications, please read the notifications help article.