Is this the sender’s fault?

Rarely. A loop is almost always in the recipient organisation’s routing, so the recipient admin has to fix it.

Will the message be delivered later?

No. It is a permanent failure, so resend once the loop is fixed.

554 5.4.14: Microsoft hop count exceeded (mail loop)

554 5.4.14Hop count exceeded, possible mail loopPermanent: retrying will not helpFrom Microsoft 365 and Outlook.com

Example

554 5.4.14 Hop count exceeded - possible mail loop

What it means

Every server that relays a message adds a Received header. Exchange Online counts them, and when the number passes its maximum hop count it stops the message and returns 5.4.14. RFC 3463 defines X.4.6 as the routing loop code; Microsoft uses its own detail value here.

The result is almost always a loop in configuration rather than a long but valid path. Two systems each believe the other is responsible for the address, so they hand the message back and forth until the counter trips.

Hybrid setups, migrations, and third party filtering services in front of Microsoft 365 are the usual places this happens.

Likely causes

Two systems both treat the domain as Internal Relay and forward unknown recipients to each other.
A mail contact or mail user whose external address points back to the same tenant.
A forwarding rule in one mailbox sends to another mailbox that forwards back.
An inbound gateway and an outbound connector send the same domain to each other.

How to fix it

Open the NDR’s original headers and read the Received lines from the bottom up to see which two hosts repeat.
Check the recipient object: a mail user or contact must not have a target address in a domain that routes back to the tenant.
Review accepted domain types and connectors on both systems so only one side is authoritative for each address.
Remove any circular forwarding rules or redirects in the mailboxes involved.

Checks that apply

Your domain or sending IP (fills in the checks below)

Email Header Analyzer Parses the Received chain and shows the repeating hosts, which is the fastest way to see where the loop sits.
MX Lookup Shows where the domain’s mail really enters, so you can tell which system should be authoritative.

FAQ

Is this the sender’s fault?: Rarely. A loop is almost always in the recipient organisation’s routing, so the recipient admin has to fix it.
Will the message be delivered later?: No. It is a permanent failure, so resend once the loop is fixed.

Microsoft 365 delist portal Outlook.com SNDS

All 191 bounce codes