That's a great question! Emails bounce as non-existent if there are any typos or the email address is no longer in use. We would suggest double-checking for any obvious typos. While on the other hand, contacts that bounce as suspended after repeatedly bouncing as non-existent. A contact can become suspended from any Constant Contact account sending email to it. For both types of bounces we would suggest contacting them outside of Constant Contact to see if they are still good. In fact, here's an article we have that goes over the different types of bounces and how they can be resolved.
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