Forwarding rules gone wrong: loops, lost mail and fixes

A forwarding rule takes a minute to set up and can run for years without anyone looking at it. That is exactly why it fails so quietly. A customer says they never got a reply. A colleague says a message from last month never reached them. Your inbox suddenly holds four hundred copies of the same automatic message. In each case, somewhere there is a rule that nobody remembers writing.

This guide covers the most common ways forwarding goes wrong, how to recognize each one, and how to set rules up so they stay predictable. It is general guidance. The exact menus differ between mail providers, so use the ideas here with your own provider's documentation.

First, know which kind of forwarding you have#

"Forwarding" covers several different things, and the failures depend on which one you use.

• A server-side forward is set in your mail account. The provider passes new mail to another address automatically, with or without keeping a copy.
• A rule or filter is a conditional version: "if the sender is X, send a copy to Y."
• An alias or group address delivers one address to many mailboxes. Strictly it is not forwarding, but it behaves similarly. See shared inbox versus distribution list versus group alias for the differences.
• A client-side rule runs only while a mail app is open on someone's computer. It stops when the laptop is closed, which is a failure by itself.

Prefer server-side rules over client-side ones. A rule that only works when a laptop is open will lose mail the first time someone is on holiday.

Problem 1: Mail loops#

What happens. Address A forwards to address B, and B forwards back to A, directly or through a third address. Each pass adds a header line. After enough passes, a mail server notices something is wrong and refuses the message with an error such as "too many hops". Before that happens you may get hundreds or thousands of copies, and the sender may receive a bounce they do not understand.

How it starts. Usually through an innocent change. Someone sets up an alias for the support address, then someone else forwards the alias back to the original mailbox "to keep a copy". Or a vacation auto-reply answers an automatic message, which triggers another automatic reply.

How to spot it. Look for a sudden burst of identical mail, a mailbox that fills overnight, or bounce messages mentioning hops or loops. In a message's full headers, many repeated "Received" lines from the same servers are a clear sign.

Fix.

1. Stop the loop first: turn off the rule or the alias that is sending mail back.
2. Draw the path on paper: every address and where it forwards. Loops are easy to see when you write them down.
3. Keep forwarding in one direction. A good habit is to make one address the "home" and let all other addresses point to it, never the other way around.
4. For automatic replies, avoid replying to messages that are themselves automatic. Our post on handling out-of-office replies explains the signals systems use to tell the difference.

Problem 2: Forwarded mail lands in spam, or never arrives#

What happens. You forward a message to a personal inbox, and the receiving provider treats it with suspicion. The most common reason is how email authentication works.

When you forward a message, the new sending server is yours, not the original sender's. The receiving server checks whether that server is allowed to send mail for the original sender's domain, and it is not. This is the classic way SPF checks fail on forwarded mail. The DKIM signature, if it was not broken by the forward, can still pass, and many domains rely on DKIM and DMARC for this reason. If the forward alters the message, for example by adding a footer or changing the subject, the signature can break too, and then neither check passes.

The result: a message that passed everything when it arrived at you may fail on the way out, and the final inbox may put it in spam or refuse it.

How to spot it. A customer-reported message never turns up in the forwarded inbox, or turns up in spam. Look at the message headers on the receiving side for authentication results.

Fix.

• Prefer forwarding that keeps the message untouched, so the original signature stays valid.
• Use a forwarding service that rewrites the envelope sender in a way receiving servers accept; large providers and hosting platforms do this, and it is worth asking yours.
• Better still, avoid forwarding for important mail. Give people access to the mailbox itself rather than copying the mail elsewhere. The reasoning is in why forwarding team mail to personal inboxes breaks at scale.
• If you run a domain with strict policies, read how mailing lists and forwarders interact with them in DMARC, forwarding, mailing lists and ARC, and the basics in SPF, DKIM and DMARC in plain English.