What the receiver is telling you

The message passed through too many servers — more than the limit of 30 or so Received headers — which means it was going round in a loop. The usual cause is on the recipient side: a mailbox forwarding to an address that forwards back, a connector that routes mail to a server that routes it to the connector, or a domain listed as authoritative in two tenants. Occasionally it is a sender-side relay misconfiguration. Either way the loop ate the message.

What a hard bounce means for your reputation

Receivers count hard bounces. Gmail and Microsoft both treat a sender whose mail is addressed to people who do not exist as a sender who did not get those addresses from the people themselves, which is the definition of unsolicited. The working threshold is two percent of a send: under that is noise, above it starts to cost domain reputation, and above five percent a fresh domain can be throttled after a single campaign. The fix is upstream — verify before sending — not downstream.

What to do

1. If the recipient is a real contact, tell them: their forwarding rule or connector is broken, and everything sent to them is failing.
2. Check your own Received headers in the bounce — if the loop is between your relay and your tool, fix the relay.
3. Remove from the current sequence; add back once the loop is fixed.

don't Do not keep sending. Every attempt takes the same loop and produces the same bounce.

Decode the whole bounce

Paste the complete failure notice — every code and the receiver's wording — and get each part explained together.

Common questions