550 5.7.26: This email has been blocked because the sender is unauthenticated

This email has been blocked because the sender is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM.

Why it happens

• Your domain has no SPF record, or the record does not list the service you send through.
• DKIM signing is not switched on in your sending service.
• Another wording of the same code: "not accepted due to domain's DMARC policy". Then your DMARC record told Gmail to reject mail that fails.
• The message was forwarded, which broke SPF, and there was no DKIM signature to fall back on.

What to do

1. Run your domain through the SPF, DMARC and DKIM checker.
2. Add your sending service to the SPF record.
3. Turn on DKIM in your sending service and publish the DNS record it gives you.
4. Bulk senders (Google counts from about 5,000 messages a day to Gmail) need SPF, DKIM and a DMARC record.

Don't confuse it with

550 5.7.27 is SPF alone, 550 5.7.30 is DKIM alone, and 421 4.7.26 is the same problem as a temporary slowdown. Microsoft's codes for this are 550 5.7.23 and 550 5.7.509.

Questions

Will cleaning my list fix it?

No. Every Gmail address refuses the message until authentication passes.

I send from a @gmail.com address through another service. Is that the cause?

Very likely. Only Google's own servers can pass authentication for gmail.com. Send from a domain you own.

Got a different bounce?

Paste the whole message into the bounce code lookup. It reads the code and the words and tells you whose move it is.

Related codes

• 550 5.7.23 Rejected because of Sender Policy Framework violation (Microsoft 365)
• 550 5.7.509 Sending domain does not pass DMARC verification (Microsoft 365)
• 550 5.7.25 The sending IP address doesn't have a PTR record (Gmail)

Free tools

• Bounce code lookup
• SPF, DMARC and DKIM checker
• Email list checker