A 550 5.4.14 reply means the message passed through too many relay hops and the server stopped it as a probable mail loop. This code is not defined in RFC 3463, whose routing family (X.4.x) ends at X.4.7; it is a provider extension that Google and Microsoft use for a hop-count-exceeded condition. Something in your routing is forwarding the message back into itself.
The enhanced status code 5.4.14 is not part of RFC 3463, whose routing and network family stops at X.4.7 ("Delivery time expired"). It is also absent from the IANA SMTP Enhanced Status Codes registry. Google Workspace and Microsoft use it to mean the message exceeded the allowed number of relay hops, which almost always signals a mail loop.
Why a loop happens
Every time a mail server hands a message on, it adds a Received header. When two systems keep forwarding to each other, that count climbs until a server gives up and returns 5.4.14. Typical setups behind it:
Two forwarders point at each other, for example a domain that forwards to an alias that forwards back.
A mailbox rule auto-forwards to an address that routes the mail back to the same mailbox.
An MX record and an internal relay disagree about which one is the final destination.
How to read the trail
Open the bounced message and count the Received headers. A healthy path has a handful; a loop shows the same two hosts alternating many times. The pair that repeats is your loop.
Common questions
Is 550 5.4.14 in the SMTP RFCs?→
No. The RFC 3463 routing family ends at X.4.7 and the IANA registry does not list 5.4.14. Google and Microsoft added it for a hop-count-exceeded loop.
What makes a hop count climb?→
Each relay adds a Received header. Two systems forwarding to each other push the count up until a server stops the loop with 5.4.14.
How do I find the loop?→
Count the Received headers in the bounce. The two hosts that alternate over and over are the forwarding pair causing it.
Is 550 5.4.14 permanent?→
Yes. The leading 5 marks it permanent, and the message keeps looping until you break one side of the forwarding chain.