550 5.7.1: Delivery not authorized, message refused
550 5.7.1 means the receiving server refused your message for policy reasons. The recipient exists, but the server decided not to accept this message from you. The wording after the code tells you why.
- Bounce type
- Hard bounce (permanent)
- Usually caused on
- Either side, read the text
- Terlido report label
- Blocked by the provider
What the standard says
RFC 5321 defines 550 as "mailbox unavailable", which includes refusals for policy reasons. RFC 3463 defines 5.7.1 as "Delivery not authorized, message refused": the sender is not allowed to send to this destination. This can come from filtering by host or by recipient. The 5 makes it permanent.
What the mailbox providers say
- Gmail (550 5.7.1): “This message is likely unsolicited email...” Source
- Gmail (550 5.7.1): “The user or domain that you are sending to (or from) has a policy that prohibits the email...” Source
- Microsoft 365 / Outlook (5.7.1): “Delivery not authorized” Source
- Microsoft 365 / Outlook (5.7.1): “Unable to relay” Source
- Microsoft 365 / Outlook (5.7.1): “Client was not authenticated” Source
Common causes
- The receiver's spam filter judged the message as unsolicited, based on content, sender reputation, or both.
- The recipient's organization has a rule that blocks mail from outside, from your domain, or from certain senders.
- On Microsoft, the recipient is a group or mailbox that only accepts mail from approved senders.
- Your sending server was not authenticated or tried to relay mail it was not allowed to relay.
- A blocklist entry for your domain or IP.
- A link in the message that points to a domain with a bad record, which causes the filter to refuse it.
How to fix it
- Read the full text. It often names the reason, for example a policy or an authentication failure.
- Check your domain and IP against blocklists and check SPF, DKIM and DMARC.
- Review the copy for spam signals such as heavy links, all caps and pushy claims.
- If one company keeps refusing you, accept that their policy blocks you and do not push more mail at them.
- Send a plain, short test message to the same recipient. If it passes, the problem is in the content of your campaign copy.
- Remove the contact from the sequence if the recipient's policy blocks outside senders. Their rule will not change for you.
Is it a deliverability problem?
It depends on the text. A policy block at one company is theirs, not yours. A message that is likely unsolicited, or the same code from many different receivers, is a sender-side problem and a sign of reputation damage. Compare the code across providers to tell which case you have. The fastest way to tell the cases apart is to count. One refusal with a policy text is a single company. Many refusals with the same unsolicited wording across different receivers say your sending is judged as unwanted. Note that Terlido blocklists the recipient on this code in both cases.
How Terlido handles it
When a bounce message for this code arrives in a connected mailbox, Terlido reads the status code. An enhanced code that starts with 5 is a hard bounce. In the campaign Reports tab this one is grouped as "Blocked by the provider".
On a hard bounce Terlido adds the address to the workspace blocklist with the reason "hard bounce", so no campaign emails it again. The lead stops in that campaign as bounced, its lead score goes down by 15, and the bounce counts toward the campaign's bounce rate.
Once a campaign has at least 20 sends, it pauses itself when hard bounces divided by sends goes over the campaign's maximum bounce rate. The default is 5%. If the cause was on your side and you fixed it, you can remove the entry on the Blocklist page of your account settings and try again.
Terlido treats every 5.x.x code as a hard bounce, so it blocklists the recipient even when the real cause is your own sending setup. After you fix the sending side, remove the entry from the blocklist if you want to contact that person again.
Check the domain behind the bounce
The free domain checker runs live SPF, DKIM, DMARC, MX and blocklist lookups and explains each result in plain English.
Free tools that help
- Email spam checkerReview advisory wording and formatting signals.
- Email blocklist checkerCheck a bounded set of named public blocklists.
Related error codes
- 554 5.7.1: Relay access denied or client host rejected
- 421 4.7.0: Temporarily deferred, often for low reputation
- 550 5.7.26: Blocked because the sender is unauthenticated
- 550 5.7.23: SPF validation failed