"550 5.7.1" email bounce

The bounce is an email titled "Delivery Status Notification (Failure)", "Undeliverable" or "Mail delivery failed". The 5 means it won't try again, and the sentence after the code says which rule.

How to read the code

Open the bounce and look for a line like 550 5.7.1 … or Remote server returned '550 5.7.1 …'. It's often in a section called "Diagnostic information" or "Technical details".

First digit 5 (550, 554)

Permanent. Your email wasn't delivered and won't be retried. Something has to change before sending again.

First digit 4 (421, 450)

Temporary. Your email provider keeps retrying, often for four or five days, and only bounces it if every try fails.

x.7.x

Security or policy: the receiving server's rules said no.

x.1.1

That mailbox doesn't exist: usually a typo, or the person has left.

x.2.2

Their mailbox is full. Nothing's wrong at your end; try another way to reach them. (Microsoft 365 also uses 5.2.2 when you have sent too many emails in a day.)

The ones you're most likely to see

550 5.7.1

"Delivery not authorized, message refused." Read the words after it. Common ones: "This message is likely unsolicited email" (Gmail thinks it's spam), "The user or domain that you are sending to (or from) has a policy that prohibits the email" (their organization blocks it, for example a group that only takes email from its members), and "Unable to relay" or "Client was not authenticated" (your email program's outgoing settings are wrong, so ask whoever set it up).

554 5.7.1

The same family. 554 means the server rejected the whole message; the reason is again in the words after it.

550 5.7.26 (Gmail)

"This email has been blocked because the sender is unauthenticated" or "Unauthenticated email from … is not accepted due to domain's DMARC policy." Gmail couldn't confirm the email came from your domain. Run Email check.

5.7.23 and 5.7.509 (Microsoft 365)

"The message was rejected because of Sender Policy Framework violation", or "… does not pass DMARC verification and has a DMARC policy of reject." The same problem as Gmail's 5.7.26: your domain's sender settings.

"banned", "blocked", "blocklist", "reputation"

The address of the server your email leaves from has a bad name, often because another customer on the same shared server sent spam, or one of your accounts was hacked. Microsoft's versions are 5.7.511 and 5.7.606 to 5.7.649.

421 4.7.0 (Gmail)

"Try again later", or "This message is suspicious due to the very low reputation of the sending IP address". Temporary, so it retries. If you see it often, treat it like a blocklist problem.

What to do

Sort it out within the week if it's one person, and the same day if your email bounces for lots of people: customers think you're ignoring them.

1. Only one person or organization bounces? It's usually their rule, not your settings. Call them, or send from another address, and ask them to let your address through.
2. Bouncing at Gmail, Outlook or Yahoo for lots of people, or the bounce mentions SPF, DKIM, DMARC or "unauthenticated"? Run Email check on your domain. SPF (the list of servers allowed to send as you), DKIM (a signature on each email) and DMARC (your rule for fakes) are explained in Stop your emails going to spam. Send the results to whoever runs your email.
3. Mentions a blocklist, a ban or a bad reputation? That's the email provider's server, and only they can get it removed. Forward them the whole bounce. While you wait, look in your Sent folder for emails you didn't write: if there are any, your account may have been broken into, so change its password and turn on two-step login.
4. Only emails from your website bounce (contact form, order emails, WordPress password resets)? Those often leave from the web server, which your SPF list doesn't name. Ask your web person to send them through your real email service.

Bounces for emails you never sent are a different problem: see Bounces for emails you didn't send.