What every campaign email carries
Every campaign email automatically gets a footer: *"You are receiving this email because you are a customer of [your company]"* plus an Unsubscribe link. You don't add it and you can't remove it. It's injected at send time.
What happens when a customer unsubscribes
Fail-closed behavior
If the system ever cannot generate working unsubscribe links, because of a configuration problem, the campaign refuses to send and is marked Failed rather than going out without an opt-out path.
That's deliberate CAN-SPAM posture: a failed campaign is recoverable, non-compliant mail is not.