SMTP 550 5.7.27
SPF authentication failed (Gmail)
SMTP 550 5.7.27 means Gmail blocked the message because it did not pass SPF. The IP that sent it is not authorised in the SPF record of the envelope sender's domain. Add the sending service to your SPF record, or send through a service that is already in it.
Should you remove the address?
Keep the address; fix the sending side
Keep it. SPF is your domain's setup.
Block. Retrying changes nothing until the cause on your side is fixed.
How it appears
550 5.7.27 This message was blocked because it didn't pass SPF authentication.
421 4.7.27 Your email has been rate limited because SPF authentication didn't pass for this message.
What causes it
- A new sending service was never added to the SPF record.
- The SPF record has a syntax error, or exceeds the 10-lookup limit and returns a permerror.
- The domain has more than one SPF record, which makes SPF fail.
- Mail is being forwarded through a server that is not in your SPF record.
How to fix it
In order — try the first one first.
- Look up your SPF record and check that every service that sends for you is included.
- Merge multiple SPF records into one and reduce lookups if you are over 10.
- Set up DKIM as well, so authentication does not depend on SPF alone.
Who sends it
Gmail.
Gmail
Reply text This message was blocked because it didn't pass SPF authentication. Gmail requires bulk email senders to authenticate their email with SPF.
The reply includes the IP that failed.
Common questions
- What does SMTP 550 5.7.27 mean?
- SMTP 550 5.7.27 means Gmail blocked the message because it did not pass SPF. The IP that sent it is not authorised in the SPF record of the envelope sender's domain. Add the sending service to your SPF record, or send through a service that is already in it.
- Should I retry a 550 5.7.27 error?
- No. 550 is a permanent failure, so the message will never be delivered no matter how many times you try. Remove the address — repeated sends to permanent failures are one of the main signals mailbox providers use to throttle a sender.
- How do I fix SMTP 550 5.7.27?
- Look up your SPF record and check that every service that sends for you is included.
- Is 550 5.7.27 a hard bounce or a soft bounce?
- 550 5.7.27 is a block rather than a classic hard or soft bounce. The rejection is about your sending (authentication, reputation, content or the recipient's policy), not about whether the address exists. Retrying changes nothing until the cause on your side is fixed.
- Should I remove the address after a 550 5.7.27 bounce?
- Keep it. SPF is your domain's setup.
- Will verifying my list prevent 550 5.7.27?
- Not directly. This code is about your authentication, IP or domain reputation, so verification will not lift it. A clean list still matters, because mailbox providers weigh how often a sender hits addresses that don't exist.
Clean the list before the next send.
A bounce report only tells you about the addresses you already mailed. The rest of the list has the same kinds of dead mailboxes in it, and each one becomes a hard bounce recorded against your domain the moment you send.
- A real SMTP mailbox check on every row, before anything is sent
- Dead and mistyped addresses split out, so they never reach your sending tool
- Catch-all, disposable and role addresses flagged alongside
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Other 550 replies
- 550 5.1.1 — mailbox does not exist
- 550 5.7.1 — message refused by policy
- 550 5.4.1 — recipient address rejected, access denied
- 550 5.2.1 — mailbox disabled, not accepting messages
- 550 — mailbox unavailable
- 550 5.1.0 — address rejected, other address status
- 550 5.1.10 — recipient not found (Microsoft 365)
- 550 5.1.8 — bad outbound sender (Microsoft 365)
- 550 5.4.5 — daily sending limit exceeded (Gmail)
- 550 4.4.7 — message expired in the queue
- 550 5.7.25 — no PTR record for sending IP (Gmail)
- 550 5.7.26 — blocked, sender unauthenticated (Gmail)
- 550 5.7.28 — unusual rate of unsolicited mail (Gmail)
- 550 5.7.23 — SPF violation (Microsoft 365)
- 550 5.7.124 — sender not in allowed senders list (Microsoft 365)
- 550 5.7.509 — DMARC failed, policy reject (Microsoft 365)
- 550 5.7.606-649 — banned sending IP (Microsoft)
- 550 5.7.705 — tenant has exceeded threshold (Microsoft 365)
- 550 5.7.708 — traffic not accepted from this IP (Microsoft 365)
- 550 5.7.750 — blocked from sending from unregistered domains (Microsoft 365)
- 550 5.7.515 — authentication required for high-volume senders (Outlook.com)
- 550 5.7.1 S3140 — IP range on Outlook.com block list
- 550 5.5.0 — Mailbox unavailable (Outlook.com, Hotmail)
- 550 5.7.12 — Sender not authenticated by organization (Microsoft 365)
- 550 5.7.513 — Blocked by the recipient's own block list (Microsoft 365)
Sources
- Source: Google Workspace Admin Help: Gmail SMTP errors and codes
- Source: RFC 3463: Enhanced Mail System Status Codes
Provider wording is quoted only where the provider publishes it.
Last reviewed 2026-10-22