550 5.7.1 — blocked

The receiving server permanently refused your message on policy or reputation grounds. Here is what 550 5.7.1 blocked means, what it costs you, and how to get your mail accepted again.

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What it means

The cause.

The reply has two parts. The 550 is an SMTP status code from RFC 5321: a permanent failure, meaning the receiver will not accept this message and retrying it unchanged will not help. The 5.7.1 is an enhanced status code from RFC 3463, and its class 5.7 is "Security or Policy Status". RFC 3463 defines 5.7.1 precisely: "The sender is not authorised to send to the destination. This can be the result of per-host or per-recipient filtering." In plain terms, the receiver decided your mail was not allowed and refused it at the door.

The word "blocked" is the receiver telling you which flavour of 5.7.1 you hit. Google returns 550 5.7.1 with wording such as "this message is likely suspicious due to the very low reputation of the sending IP address" or "sending domain", "this message is likely unsolicited email", or a message that your mail "violates" a specific domain policy. Microsoft returns 5.7.1 when the sending IP is on its blocklist ("Client host [address] blocked using Blocklist 1"), when authentication is required to relay, or when a recipient rule refuses external senders. The common thread is reputation, authentication or an explicit policy rule — not a typo in the address.

Because it is a 5xx code it is permanent for this attempt: the receiver is not asking you to wait and try later, it is telling you the message was rejected outright. What varies is whether the underlying cause is your sending reputation, a blocklist listing, a broken authentication set-up, or a deliberate rule on the recipient side.

Why it matters

The impact.

A 550 5.7.1 block is a hard bounce, so the message never reaches the inbox or the spam folder — it is refused before delivery. Every hard bounce is logged against your sending reputation, and a rising bounce rate makes the next message more likely to be blocked too, which is how a handful of rejections turns into a domain-wide problem.

Reputation-based blocks rarely stay isolated. Since February 2024 Gmail, Yahoo and Microsoft hold bulk senders to explicit rules — authenticate with SPF and DKIM, align under DMARC, keep spam complaints below 0.3%, and honour one-click unsubscribe. Miss those and mail that used to land starts getting throttled, filed as spam, or blocked with exactly this code. Once an IP or domain earns a poor reputation, clearing it takes consistent, authenticated, low-complaint sending over days, not a single fix.

What to do

The fix, in order.

Work top to bottom, then re-check — DNS changes take a few minutes to take effect.

1 Read the full bounce, not just the code. The text after "550 5.7.1" is the actionable part. "Low reputation", "unsolicited", "Blocklist 1", "authentication required" and "violates policy" each point at a different cause. Copy the complete diagnostic line from the bounce or the non-delivery report before you change anything, so you fix the reason the receiver actually gave.
2 Run a full deliverability test. Send a message through the email tester. It checks SPF, DKIM and DMARC, scores the content for spam signals, and reports whether your sending IP or domain is listed on the major blocklists — the three things that produce a 550 5.7.1 block. That tells you which layer is failing instead of guessing.
3 Fix authentication and alignment. Publish a valid SPF record that covers every service you send through, sign your mail with DKIM, and add a DMARC record so the two align with your From: domain. Unauthenticated bulk mail is now blocked outright by Gmail and Yahoo, and gaps here are a frequent trigger for this code.
4 Check and clear blocklist listings. If the bounce names a blocklist, or the tester shows your IP or domain listed, that listing alone can produce 550 5.7.1. Use each list operator's own delisting process — Microsoft accepts requests at [email protected] — and fix the cause of the listing (compromised account, open relay, spam complaints) before you request removal, or you will be relisted.
5 Reduce complaints and volume, then warm up. If the block is reputation-based, remove unengaged and invalid addresses, add one-click unsubscribe, and keep spam complaints under 0.3%. Lower your send volume and rebuild slowly with engaged recipients so the receiver sees a consistent, wanted stream rather than a spike.
6 If it is a recipient rule, contact them. When the wording says the message violates a specific domain policy or that you lack permission to send to the recipient, the block is a deliberate rule on their side. You cannot fix that yourself — ask the recipient or their email admin to add you to their accepted senders.

Confirm the error is gone.

Run a full check on your domain — it reads SPF, DKIM and DMARC live and tells you, in plain terms, whether the thing behind this error is fixed. Free, no account.

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Common questions

About this error.

What does 550 5.7.1 — blocked mean? It is a permanent SMTP rejection on security or policy grounds. RFC 3463 defines 5.7.1 as the sender not being authorised to send to the destination, from per-host or per-recipient filtering. The "blocked" text means the receiver refused the message because of reputation, a blocklist listing, failed authentication, or an explicit rule.

How do I fix 550 5.7.1 — blocked? Read the full bounce text to see the stated reason, then run a deliverability test to check SPF, DKIM, DMARC, spam signals and blocklist status. Fix the failing layer — repair authentication, request delisting and clear the cause, or reduce complaints and volume — then re-test before you resume sending.

Is 550 5.7.1 — blocked permanent? The 5 in 550 marks a permanent failure, so that specific message is rejected and retrying it unchanged will not help. It is not permanent for your domain: once you resolve the reputation, blocklist, authentication or policy cause, new messages are accepted again.

Why does Gmail return 550 5.7.1? Gmail uses this code when it blocks mail for the very low reputation of the sending IP or domain, when a message looks like unsolicited spam, or when it violates a specific domain policy. Since 2024 it also blocks bulk mail that is not authenticated with SPF, DKIM and DMARC.

Why does Outlook or Microsoft 365 return 550 5.7.1? Microsoft returns 5.7.1 when the sending IP is on its blocklist (shown as "Client host blocked using Blocklist 1"), when authentication is required to relay, or when a recipient or group rule refuses external senders. For a blocklist listing, forward the bounce to [email protected] after fixing the cause.

Is 550 5.7.1 an SPF or DMARC error? Not specifically — it is a general policy and authorisation rejection, not a single authentication result. Failed SPF or DMARC is one common cause, but so are poor reputation, blocklist listings and recipient rules. Read the bounce wording and test all three layers to find which applies.

Fix the cause, and check it.

Blacklist checker → Check your domain and every sending IP against the lists receivers consult. Spamhaus ZEN removal → If a listing is behind the bounce, here is how to get your IP off it. Full domain check → SPF, DKIM, DMARC, MX and the blacklists, scored in one run.