Enhanced status code

5.4.14: hop count exceeded, possible mail loop

5.4.14 is Exchange Online’s code for a mail loop. The message was passed back and forth between servers until Exchange stopped it and sent you a bounce. The address is usually fine; the routing between the systems that handle it is not.

Updated October 10, 2026 · 4 min read

Enhanced code
5.4.14 (Microsoft; not in the IANA registry)
NDR text
Hop count exceeded - possible mail loop
On-premises twin
554 5.4.6 (Exchange Server)
Retry?
Not until an admin fixes the routing

Verification would not have caught it

A routing loop happens after a server has accepted the message, inside the recipient’s or your own mail setup. A verification can’t see that far, and resending won’t help until the routing is fixed.

5.4.14 Hop count exceeded: a message loops between Exchange Online and another mail server until Exchange stops it after 20 loops and sends an NDR

What 5.4.14 means

5.4.14 means Exchange Online found the message in a loop and stopped it. Microsoft’s article on 5.4.6 and 5.4.14 shows the NDR text:

5.4.14 Hop count exceeded - possible mail loop ATTR34

and explains: “5.4.14 indicates a mail loop or routing problem in Exchange Online.” The on-premises equivalent is 554 5.4.6 with the same text, “Hop count exceeded - possible mail loop”, which Exchange Server generates and which you meet in hybrid environments. Microsoft’s NDR reference lists both under “Routing loop detected” and adds: “By default, after 20 iterations of an email loop, Exchange interrupts the loop and generates an NDR to the sender of the message.”

The number is Microsoft’s own. The IANA registry of enhanced codes ends the routing group at X.4.7. The standard code for the same problem is X.4.6, which RFC 3463 calls “Routing loop detected”: “A routing loop caused the message to be forwarded too many times, either because of incorrect routing tables or a user-forwarding loop.”

How a mail loop happens

Every server that handles a message adds a Received: line to its header. RFC 5321 says servers “MUST contain provisions for detecting and stopping trivial loops”, and that counting these lines “has proven to be an effective, although rarely optimal, method of detecting loops”. It suggests a threshold of “normally at least 100”; Gmail stops at 50 hops (554 5.4.6 “Message exceeded 50 hops, this may indicate an email loop.”), Exchange after 20 iterations of a loop.

A loop forms when two systems each think the other is responsible for a recipient, or when rules keep passing a message on. In a bounced message’s header, you see the same few servers again and again; the email header analyzer lists every hop in order.

What causes 5.4.14

Microsoft names these causes:

CauseWhat goes wrong
Accepted domainExchange Online accepts the message because of the recipient’s domain, “but then couldn’t correctly route the message to the recipient.”
Hybrid inboundMail is routed by DNS instead of a smart host: the MX record points to Exchange Online, and the connector to your on-premises servers uses DNS routing, so it looks up the MX and sends the message straight back to Exchange Online
Hybrid outboundAll outbound mail goes via on-premises: with RouteAllMessagesViaOnPremises on, a missing or wrongly scoped connector from Office 365 to your servers breaks the route
ForwardingMailbox rules that forward automatically, for example two mailboxes forwarding to each other

The accepted domain type matters here. Microsoft’s guide to accepted domains defines “Authoritative” as: email for unknown recipients is rejected. “Internal relay” sends mail for recipients Microsoft 365 doesn’t know on to your own mail server, and needs a connector to it. Set to internal relay without a working route elsewhere, mail for an unknown address can come straight back.

How to fix 5.4.14

If you’re the sender: Microsoft says “these issues can only be fixed by an Exchange Online admin and not the average email sender.” Forward the NDR to your admin, or to the recipient’s admin if the bounce came from their organization.

If you’re the Exchange Online admin:

  1. Check the accepted domain. In the Exchange admin center, under Mail flow > Accepted domains, make sure the recipient’s domain is Authoritative if all its mailboxes are in Microsoft 365. Microsoft: “You should not select this option if all of the recipients for this domain are in Microsoft 365 or Office 365” (about internal relay).

  2. Hybrid inbound routing. Make sure the connector to your on-premises organization uses smart host routing to your hybrid server, not DNS. The simplest fix, according to Microsoft, is to run the Hybrid Configuration Wizard again.

  3. Hybrid outbound routing. Check the connector setting:

    Get-OutboundConnector -Identity "<Connector Name>" | Format-List Name,RouteAllMessagesViaOnPremises

    If it’s True, you need a connector from Office 365 to your email server with the connector type On-premises that isn’t scoped to any accepted domains.

  4. Look for forwarding loops. Check both mailboxes’ rules and forwarding settings, and any mail flow rules that redirect messages.

  5. Check the MX records of the domain with the MX lookup. The MX record guide explains how they decide where mail goes.

The list of SMTP error codes covers the rest.

Would email verification have prevented it?

No. A loop happens after a server has accepted the message, inside the routing of the organizations involved. An email verification asks the first server whether it accepts the address, and that server says yes. Our Microsoft 365 SMTP settings page helps when the loop starts at an app or device that sends through Microsoft 365.

Fixed the loop? Keep your list clean too

A mail loop is about routing, not the address. Bounces from addresses that don’t exist are a separate problem: email verification finds them before you send, without sending anything.

Check one address Clean a whole list

Frequently asked questions

What does error 5.4.14 mean?

Exchange Online detected a mail loop: the message went around between servers too many times. The NDR says “5.4.14 Hop count exceeded - possible mail loop ATTR34”. Microsoft says the likely causes are accepted domain settings and, in hybrid setups, misconfigured connectors.

What is the difference between 5.4.6 and 5.4.14?

Both are routing loops. On-premises Exchange Server reports 554 5.4.6, which you see in hybrid environments; Exchange Online reports 5.4.14. Gmail uses 554 5.4.6 too, with “Message exceeded 50 hops”.

How do I fix 5.4.14?

As a sender you can’t; forward the NDR to the admin of the domain that bounced it. The admin checks that the recipient’s domain is an authoritative accepted domain in Exchange Online, reviews the hybrid connectors, and looks for forwarding rules that send mail back and forth.

Can a forwarding rule cause 5.4.14?

Yes. Microsoft’s NDR reference says to check the sender’s and recipient’s mailbox rules for automatic forwarding: two mailboxes that forward to each other create a loop.

Related codes and guides

550 5.7.708550 5.7.708 means Microsoft 365 blocked your organization’s outgoing mail because the sending IP has a low reputation. See who can fix it and what to do first. 554 5.7.0554 5.7.0 means the server ended the session because your app sent commands without logging in. See Google’s 5.7.0 replies and how to set up SMTP login. 550 5.7.0550 5.7.0 means Google’s SMTP relay refused to send your mail: unknown IP or domain, or a suspended account. See the three messages and how to fix each one. 550 5.7.520550 5.7.520 means Microsoft 365 blocked an automatic forward to an outside address. See why the policy exists, who can change it and how admins allow it safely. 4.4.74.4.7 means your server kept retrying until the message expired. Learn how to read “550 4.4.7 QUEUE.Expired” in Microsoft 365 and how to find the real cause. Microsoft 365 email settingsOffice 365 SMTP settings: smtp.office365.com, port 587, STARTTLS, TLS 1.2+. Microsoft 365 IMAP and POP3 servers, OAuth, SMTP AUTH status and error fixes.

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