Hi @EMJC ,    That looks like an automated response that was sent out confirming their registration when they signed up via the form inside of your email campaign - the email you sent out through Constant Contact uses a third-party form. My guess would be that they are mistyping their email address into that form, hence the undeliverable notification, but I would advise reaching out to that third-party to confirm. Their email in Constant Contact is accurate, otherwise they wouldn't be able to fill out that form at all. You can also feel free to check the engagement history for that specific contact to confirm that they are being received/opened.  ... View more