554 5.4.14: hop count exceeded, possible mail loop

What it means

Each server that handles a message adds a hop. When the count passes the limit, Exchange assumes the message is caught in a loop and returns it; by default it breaks a loop after 20 iterations. Exchange Online reports this as 5.4.14, often with ATTR34; on-premises Exchange Server, seen in hybrid setups, reports 554 5.4.6. Microsoft applies the same guidance to related codes up to 5.4.20. The usual causes are configuration errors: an accepted domain set up wrongly, hybrid connectors that route mail back and forth, forwarding rules that point at each other, or synced users whose domains are not verified in Microsoft 365.

Common causes

• The recipient domain is not configured as an authoritative accepted domain in Exchange Online.
• A hybrid connector uses DNS routing instead of the on-premises smart host, sending mail straight back to Exchange Online.
• The connector from Office 365 to the on-premises server is scoped to accepted domains or is not of the On-premises type.
• Synchronized users have domains that are not verified in Microsoft 365.
• Forwarding rules on the sender and recipient mailboxes pass messages back and forth.

How to fix it

1. 01Check sender and recipient mailbox rules for automatic forwarding that could form a loop.
2. 02Admins: verify the recipient domain under Mail flow > Accepted domains.
3. 03In hybrid setups, rerun the Hybrid Configuration Wizard, or check that the hybrid connector routes to the on-premises smart host and is not scoped to accepted domains.
4. 04Add and verify in Microsoft 365 any domains used by synchronized users.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

• Fix NDR error 5.4.6 or 5.4.14 in Exchange Onlinelearn.microsoft.com
• Primary SMTP addresses are displayed incorrectly or secondary SMTP addresses are missinglearn.microsoft.com
• Email nondelivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com