When an email is sent off to its recipients, there are times when we get a message back from the receiving server. This is what constitutes a bounce. They are typically broken down for you into various categories:
Non-existent: The recipient no longer has that email address and it has been removed.
Undeliverable: The internet service provider did not respond. Check these ones for possible spelling errors after the @ sign. (i.e. gmial.com or yaho.com)
Suspended: The address has consistently bounced as non-existent and is on a hold.
Blocked: The service provider has blocked the email from Constant Contact.
Vacation/Auto-Reply: You may still get some of these messages sent to your inbox; however since we received a message back from the server, it is listed under "Bounces"
Other: We could not decipher the bounce message that we received.
I hope this information help. Let us know if you need any further information.