How to handle bounced unsubscribes?

Your subscriber sends an unsubscribe email to your list-management address and it bounces. They think they've opted out. You keep sending. The next email they get is the one they report as spam. That's the scenario you're preventing when you audit how your unsubscribe processing actually handles failed requests.

The most common cause of bounced unsubscribes is a misconfigured List-Unsubscribe header. If your ESP generates a mailto: link that points to an address that isn't set up to receive external mail (or routes through a mailbox that's full, deactivated, or rejecting inbound messages), unsubscribe requests via email bounce silently. The subscriber gets no confirmation, and your ESP never processes the opt-out. The fix is to confirm that address accepts inbound mail and that your ESP automatically processes requests sent to it, not just stores them.

Hard-bouncing addresses are a different but related issue. If an address is already hard bouncing, it should be on your suppression list before any unsubscribe request enters the picture. A hard bounce means the address doesn't exist or is permanently rejecting mail. You shouldn't be waiting for an unsubscribe request from an address that can't receive your emails.

For soft bounces, the address still exists but is temporarily unavailable. Your ESP retries these automatically, and most unsubscribe requests via the one-click or landing page method don't bounce at all since they're web requests, not email. But if an email-based unsubscribe request soft bounces repeatedly, treat it as a confirmed opt-out. The rule is simple: when in doubt, suppress. Sending to someone who's been trying to unsubscribe will damage your sender reputation more than losing the contact ever would.

To check your setup: send yourself a test email, find the List-Unsubscribe header, extract the mailto: address, and send a message to it from an external account. If you get a confirmation and your ESP processes the opt-out, you're set. If it bounces or disappears with no action, that's the gap to fix.