Bounce message 550: user unknown, relay denied and rejected, what each one means

You sent an e-mail and got back a message that says “Mail Delivery Subsystem”, “Undelivered Mail Returned to Sender” or “Delivery Status Notification (Failure)”. That is a bounce, and what matters is at the bottom of it, on a line like 550 5.1.1 .... The first digit says a lot: 5 is a permanent failure (no point trying again without changing something), 4 is temporary (see error 421 and 451).

The 550s you will see most

What the line says (wording varies from server to server) What it means Who has to act
550 5.1.1 user unknown; the account does not exist The destination address does not exist, or has a swapped letter. You: confirm the address with the recipient another way.
552 5.2.2 or mailbox full The recipient’s mailbox has reached its quota. The recipient. See your mailbox is full.
550 5.7.1 relaying denied; relay access denied The server your program sent to will not send outward without identification. It happens when sending authentication (SMTP) is switched off in the program. You: switch authentication on and check the ports. See why your e-mail is not sending or receiving.
550 5.7.1 blocked, rejected by policy, blocklist The recipient’s server refused your sending address because of reputation, or because the IP is on a blocklist. Both. See your server IP on a blocklist.
550 5.7.26 unauthenticated (SPF/DKIM) The destination demands that the domain proves the message is yours, and the proof did not pass. You: SPF, DKIM and DMARC.
554 or 550 with the word DMARC The domain published a reject rule and the message failed it. You: creating your DMARC record.

How to read a bounce, step by step

1 Go to the bottom of the message and look for the technical part: it usually has “host ... said” or “Remote server returned”. That line is the reply of the server that refused, word for word.
2 See which server refused. If it was the recipient’s, the failure is the destination’s or your reputation. If the refusal comes from the server you sent through, it is your program’s configuration.
3 Compare: does it fail for one recipient only, or for all? One: address, full mailbox or their block. All: something on your side (authentication, SPF, blocklist).
4 If you need to know what happened after sending, cPanel keeps the trail: Track Delivery shows the reply to each attempt.
Getting bounces for messages you never sent? Someone used your address as a fake sender, and the bounces from that person’s errors landed on you. In general there is no break-in to your account. What protects your domain in the long run is DMARC: creating your DMARC record.
Do not resend ten times. On a 5 error, resending the same does not help and, for some blocklists, even makes it worse. First change what the technical line points at.

Paste us the technical line of the bounce (the one starting with 550) and tell us the destination address.

Open a support ticket

SEE ALSO

Why your e-mail is not sending or receiving

Error 421 and 451: temporary failures, and why the message is delayed

Track Delivery: finding out what happened to a message you sent

SPF, DKIM and DMARC: why your e-mail lands in spam

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