Gmail blocked your email because the sender is unauthenticated. Here is the fix.

The bounce looks like this

550-5.7.26 This mail has been blocked because the sender is unauthenticated.
550-5.7.26 Gmail requires all senders to authenticate with either SPF or DKIM.
550-5.7.26
550-5.7.26  Authentication results:
550-5.7.26  DKIM = did not pass
550-5.7.26  SPF [example.com] with ip: [203.0.113.25] = did not pass
550-5.7.26
550-5.7.26  For instructions on setting up authentication, go to
550 5.7.26  https://support.google.com/mail/answer/81126#authentication

The lines under "Authentication results" are the diagnosis: they tell you whether SPF, DKIM or both failed for the message Gmail rejected.

How to fix it, step by step

1. 1

   Confirm which mechanism failed

   The bounce itself tells you. "SPF ... did not pass" means the sending IP is not authorized in your SPF record. "DKIM = did not pass" means the message had no valid DKIM signature for your domain. If both failed, Gmail had no way to verify the mail at all and rejected it.

2. 2

   Fix SPF: authorize everything that sends as your domain

   Your SPF record (a TXT record on your domain) must include every service that sends mail for you: your mailbox provider, CRM, invoicing tool, marketing platform. A missed include is the single most common cause of this bounce. Also check you have only ONE SPF record and stay under 10 DNS lookups - either mistake invalidates the whole record.

3. 3

   Fix DKIM: sign with your own domain

   Enable DKIM in each sending service and publish the CNAME or TXT records they give you. Signing with the vendor default (like a shared sendgrid.net identity) is no longer enough for Gmail - the signature should be for your domain.

4. 4

   Publish a DMARC record

   Gmail requires bulk senders to have at least v=DMARC1; p=none with a rua reporting address. Without it, even mail that passes SPF and DKIM can be throttled or rejected. Start at p=none to collect data, then move to p=quarantine or p=reject once reports confirm all legitimate senders pass.

Related guides

• Microsoft error 550 5.7.515 - what it means and how to fix it
• SPF permerror: too many DNS lookups - what it means and how to fix it
• DMARC p=none: what it does, what it does not, and when to move off it

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