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550 5.7.1 error: what it means and how to fix it

550 5.7.1 means the receiving server refused your message on policy grounds. The text after the code says why: spam, reputation, a block list or a setting.

Depends on the textThe same code covers different causes. Read the text after the code.

What 550 5.7.1 means

Any server5.7.1 Delivery not authorized, message refused

The standard meaning: the sender isn't authorized to send to this destination, so the message is refused for good.

Gmail550 5.7.1 This message is likely unsolicited email.

Gmail judged the message as spam. Other Gmail texts on the same code cite the very low reputation of your sending IP or domain, a rate limit, or a malformed header (a missing From or Message-ID, duplicate headers).

Gmail550 5.7.1 The user or domain that you are sending to (or from) has a policy that prohibits the email

A domain policy, on your side or theirs, blocks the message.

Microsoft 365 / Outlook550 5.7.1 Service unavailable; Client host [IP] blocked using Blocklist 1

Your sending IP is on Microsoft's block list. Microsoft's own fix: forward the bounce to [email protected] or use its delist portal.

List problem or sender problem?

Depends on the text. 5.7.1 is a policy refusal, and the policy can be about you (your IP or domain's reputation, a block list, your content) or about them (a group or mailbox that only takes certain senders). The text after the code tells you which. It's a permanent failure: the server won't accept this message if you send it again unchanged.

How to fix 550 5.7.1

  1. Read the text after 5.7.1 first. It names the reason: spam, reputation, a block list, a header problem or a setting on their side.
  2. If Gmail says unsolicited or low reputation, slow down, tighten targeting and copy, and follow Google's sender guidelines before sending more from that domain.
  3. If Microsoft names a block list, fix what got you listed, then forward the bounce to [email protected] or use Microsoft's delist portal.
  4. If Gmail says a header is wrong, fix it: one valid From, a unique Message-ID, no duplicate headers.
  5. If it's a group or mailbox setting on their side, only their admin can allow you. Microsoft says most 5.7.1s can't be fixed by the sender; for outreach, suppress the address.

In SendingOS, 550 5.7.1 shows on the conversation with its reason and code, so a problem on their side never gets mistaken for one on yours.

Not sure whether a spike in bounces is the list or the domain? Read High email bounce rate? Burned domain or bad list.

554 5.7.1550 5.7.26421 4.7.0550 5.7.511

Sources: Google: Gmail SMTP errors and codes · Microsoft: fix 550 5.7.1 · RFC 3463

Questions

Is 550 5.7.1 a hard bounce?

Yes. It's a permanent failure: sending the same message again won't get through. Fix the cause the text names, or suppress the address if it's a setting on their side.

Why does 550 5.7.1 mean so many different things?

5.7.1 is the general code for a policy refusal, so every provider uses it for several reasons. Gmail alone attaches it to spam verdicts, low reputation, rate limits and malformed headers. The text after the code tells them apart.