What is an email bounce loop?
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An email bounce loop is what happens when two mail servers or automated systems get stuck in an infinite cycle of replying to each other's error messages. Server A sends a bounce to Server B. Server B's auto-responder replies to that bounce. Server A treats that reply as invalid and bounces it back. Server B replies again. And so on.
In practice, this almost never happens anymore because modern mail servers have built-in loop detection. After a message bounces 2-3 times along the same path, the server stops processing it and drops it silently. But bounce loops used to be a real problem in the early 2000s, back when auto-responders and vacation messages didn't check whether the incoming message was already an automated reply.
Where you might still see this: misconfigured forwarding rules. Let's say you forward captain@deepcurrent.io to lighthouse@harborpost.net, and that mailbox forwards back to captain@deepcurrent.io. If a message can't be delivered at one of those stops, you've created a loop. Most mail servers will detect this within a few hops and kill the message, but it still wastes server resources and creates confusing bounce logs.
Another scenario: an ESP with a poorly configured automated reply system. If your Mailchimp or SendGrid account is set to auto-respond to bounces (don't do this), and the bounce itself came from another automated system that replies to everything, you've got a loop. Modern ESPs won't let you set this up, but custom SMTP setups sometimes miss it.
How to avoid bounce loops entirely: never send automated replies to bounce messages, never forward a mailbox to itself or create circular forwarding chains, and if you're running your own mail server, make sure loop detection is enabled (it usually is by default). If you're using an ESP, you're almost certainly fine. They handle this for you.
If you're seeing repeated bounce messages in your logs that look like they're replying to each other, check your forwarding rules and any automated reply settings. Most of the time, it's a forwarding misconfiguration rather than a true server-to-server loop.
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