SMTP 550 5.7.26
blocked, sender unauthenticated (Gmail)
SMTP 550 5.7.26 is Gmail rejecting mail that failed authentication. Google uses it when the sender passes neither SPF nor DKIM, when SPF hard-fails, and when the message fails the sending domain's DMARC policy. Gmail requires authentication for all senders, so this is blocked until your records are fixed.
How it appears
550 5.7.26 This mail has been blocked because the sender is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM.
550 5.7.26 Unauthenticated email from example.com is not accepted due to domain's DMARC policy.
550 5.7.26 The MAIL FROM domain [example.com] has an SPF record with a hard fail policy (-all) but it fails to pass SPF checks with the ip: [203.0.113.5].
What causes it
- Neither SPF nor DKIM passed for the message.
- The domain's SPF record ends in -all and the sending IP is not listed.
- DMARC failed and the domain publishes p=reject or p=quarantine.
- A sending tool sends as your domain without DKIM signing set up.
How to fix it
In order — try the first one first.
- Set up DKIM signing for every service that sends as your domain.
- Add each sending service to SPF and keep the record under 10 DNS lookups.
- Make sure the domain in the From header aligns with the SPF or DKIM domain, so DMARC passes.
- Run a domain audit to confirm SPF, DKIM and DMARC pass before sending again.
Common questions
- What does SMTP 550 5.7.26 mean?
- SMTP 550 5.7.26 is Gmail rejecting mail that failed authentication. Google uses it when the sender passes neither SPF nor DKIM, when SPF hard-fails, and when the message fails the sending domain's DMARC policy. Gmail requires authentication for all senders, so this is blocked until your records are fixed.
- Should I retry a 550 5.7.26 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 5.7.26?
- Set up DKIM signing for every service that sends as your domain.
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 5.7.27 — SPF authentication failed (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
Last reviewed 2026-09-11