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Definition
💡 An email loop is when two automated systems, usually auto-responders, keep replying to each other back and forth, over and over, without a human ever stepping in to stop it. Also called a mail loop
How two auto-replies end up talking forever
Here's the setup that usually causes it. You set an out-of-office reply. So does the person you emailed. Their auto-reply triggers your auto-reply, which triggers theirs again, which triggers yours again. Neither system knows to stop.
It sounds absurd when you say it out loud, and it kind of is. But it's a genuinely common problem, especially around holidays when half the office has vacation responders running simultaneously. A milder version happens with forwarding rules too: Account A forwards to Account B, which forwards back to Account A, creating an identical endless cycle with zero human involvement.
The real risk isn't just a messy inbox. Enough looped messages in a short window can get your account flagged by your provider's spam-prevention systems, since a sudden burst of repeat email to the same address looks a lot like spam behavior, even when it isn't.
There's also another indirect cost. If a loop runs long enough overnight, you can wake up to hundreds of identical messages clogging your inbox, burying anything actually important that arrived while you were asleep. Sorting through that mess costs you real time the next morning, on top of whatever the loop itself was supposed to be telling you.
The three ways a loop actually starts
Auto-reply loops are the classic version: two vacation responders, or one responder plus a badly configured customer support bot, talking past each other endlessly.
Forwarding loops happen when mail gets redirected in a circle across two or more accounts, none of which realize they're forwarding to something that eventually leads right back to where it started.
Mailing list loops are rarer but nastier: an auto-reply gets sent to an entire distribution list, and if enough recipients also have auto-replies on, the loop multiplies instead of just bouncing between two parties.
Gmail actually builds in some protection against the first type. Google's vacation responder documentation confirms it only replies to the same sender once every four days, which quietly kills most simple two-way loops before they spiral. Outlook doesn't have quite the same automatic guardrail for classic Automatic Replies, which is part of why loops show up more often in mixed-environment offices.
What to do if you find yourself in an email loop
If you're the one setting things up, a little caution goes a long way: never stack forwarding and auto-reply on the same account without checking the other end, since a redirect that loops back to something set to auto-reply will start the whole cycle right back up.
It's also worth testing auto-replies with a personal account first rather than a live work address, since a test loop between two real inboxes at scale is a bad way to find out your setup is wrong.
More often, though, you'll be on the receiving end of someone else's misconfigured system, and the fix there looks different.
The first sign is usually a sudden flood of identical subject lines landing one after another, and once you spot that pattern, don't reply, and definitely don't reply-all, since responding can restart or extend the loop on the sender's end.
Select the messages and delete or archive them in bulk rather than working through them one by one, and if your inbox supports it, a quick search filtered to that sender or subject line makes short work of it.
If the loop is coming from someone at your own company, a quick message to them or to IT usually resolves it fast, since they likely don't know it's happening yet. And if it's coming from an external account, most loops burn themselves out within a few days on their own, particularly against Gmail addresses, since Gmail's built-in four-day throttle usually kills the cycle before it spirals much further.