What is an autoresponder loop?
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An autoresponder loop is when two automated email systems get stuck replying to each other's replies, creating an endless back-and-forth that floods both mailboxes. Classic example: your Out of Office message triggers someone else's Out of Office, which triggers yours again, repeat forever.
This usually happens with OOO replies, but it can also fire off between vacation messages and newsletter confirmation emails, support ticket auto-replies and customer auto-confirmations, or any two systems that both reply automatically to incoming mail. If neither system is configured to detect and stop the loop, they'll keep going until someone manually intervenes or a server hits a rate limit.
Why it matters for deliverability: autoresponder loops generate massive email volume in a short time, which looks suspicious to spam filters and can trigger rate limiting or temporary blocks from receiving servers. If your domain gets caught in a loop with a major mailbox provider, you might see delayed delivery or soft bounces on legitimate mail until the issue's resolved.
How to prevent it: most modern email systems (like Google Workspace, Microsoft 365, and ESPs like Postmark) have built-in loop detection. They track the Auto-Submitted header and won't auto-reply to messages that already have Auto-Submitted: auto-replied. If you're building custom autoresponders, always check for this header before sending a reply. Also set a daily limit on how many auto-replies one address can trigger.
And if you're stuck in a loop right now, disable the autoresponder immediately and manually clear any queued replies. Then check your mail server logs to see which address triggered the loop and add that sender to your autoresponder's suppression list before turning it back on.
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