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550 5.7.26: the email is unauthenticated

The message failed the receiver's authentication checks, so it was refused. The fix is in your DNS: SPF, DKIM and a DMARC record that line up with your From domain.

Updated 4 min read

On this page (7)

550 5.7.26 is a permanent SMTP reply: failed SPF, DKIM or DMARC. This page is part of our SMTP error code reference, where every code links to the RFC and to what Gmail, Outlook and Yahoo publish about it.

Kind
Permanent: sending the same message again fails the same way
Retry by hand
No, fix the cause first
In Mailbay
bounced, address not suppressed

What 550 5.7.26 means

RFC 7372 registers X.7.26 as "Multiple authentication checks failed", returned "when a message failed more than one message authentication check" [2]. The basic code 550 makes it a permanent refusal [1].

In practice it means the receiver could not tie the message to its From domain with SPF or DKIM, and the domain's DMARC policy, or the receiver's own rules, say to refuse it.

A received message in the Mailbay dashboard with its authentication results: spf fail, dkim none, dmarc fail
What a receiving server sees in an unauthenticated message: Mailbay's dashboard shows this test message's real results, SPF fail, no DKIM signature and DMARC fail. Gmail answers mail like this with 550 5.7.26.

What Gmail, Outlook and Yahoo say

Gmail
"550 5.7.26 This email has been blocked because the sender is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM." Two other 550 5.7.26 rows cover an SPF hard fail and a domain whose DMARC policy rejects [3]. Since February 1, 2024, all senders need SPF or DKIM, and senders of more than 5,000 messages a day need SPF, DKIM and DMARC [4].
Yahoo
Does not list 5.7.26, but requires SPF or DKIM from every sender, and from bulk senders "a valid DMARC policy with at least p=none - DMARC must pass" [5].
Outlook and Microsoft 365
Uses the temporary 4.7.26 for mail over IPv6 that "must pass either SPF or DKIM validation" [6], and 550 5.7.515 for high volume senders without SPF, DKIM and DMARC.

Sources, read on October 14, 2026

  1. [1]RFC 5321, Simple Mail Transfer Protocol, section 4.2: www.rfc-editor.org/rfc/rfc5321#section-4.2
  2. [2]RFC 7372, Email Authentication Status Codes: www.rfc-editor.org/rfc/rfc7372
  3. [3]Google, Gmail SMTP errors and codes: knowledge.workspace.google.com/admin/support/troubleshooting/gmail-smtp-errors-and-codes
  4. [4]Google, Email sender guidelines: support.google.com/a/answer/81126
  5. [5]Yahoo Sender Hub, best practices: senders.yahooinc.com/best-practices/
  6. [6]Microsoft Learn, Email non-delivery reports and SMTP errors in Exchange Online: learn.microsoft.com/en-us/troubleshoot/exchange/email-delivery/ndr/non-delivery-reports-in-exchange-online

Prices change. If something here is out of date or wrong, email hello@mailbay.dev and we will fix it.

Common causes

  • No DKIM signature, or a signature for a domain other than the one in From.
  • An SPF record that does not include the service actually sending your mail.
  • Two SPF records on one domain, which makes SPF fail with an error.
  • A DMARC policy of quarantine or reject while SPF and DKIM do not align with the From domain.
  • Forwarding, which breaks SPF; DKIM is what survives it.

How to fix it

  1. Send through a provider that signs with DKIM for your own domain, and publish its DKIM record.
  2. Publish one SPF record that includes every service that sends as your domain. To merge two, see SPF, DKIM and DMARC explained.
  3. Publish a DMARC record, starting with p=none and a report address, then tighten it once reports show everything passing.
  4. Send a test email to the free deliverability test to see which of SPF, DKIM and DMARC fails and how to fix it, or send one to a Gmail address and open "Show original".

How Mailbay reports it

When a recipient's server answers 550 5.7.26 to mail you send through Mailbay, the recipient becomes bounced and you get a message.bounced event with the server's reply in response and permanent: false. The message statuses page lists every state.

A policy refusal says nothing about whether the address exists, so Mailbay does not suppress it, and it does not count toward the automatic pause, which only counts hard bounces and spam complaints.

When you add a domain, Mailbay generates the SPF include, a DKIM key with the selector mb1 and a DMARC record, checks each one live and merges an existing SPF or DMARC record instead of replacing it (domain setup). Mail you send from a verified domain is DKIM signed for that domain, which is the alignment Gmail looks for.

Mailbay checks incoming mail the same way: every received message shows its SPF, DKIM and DMARC results in the dashboard and in the API's auth_results.

Terminal output of dig TXT for the _dmarc records of gmail.com, yahoo.com and outlook.com
The DMARC records Gmail, Yahoo and Outlook publish for their own domains, read with dig on October 14, 2026: Yahoo rejects mail that fails DMARC, the other two monitor.

Related codes

  • 550 5.7.515: sender rules at Outlook.com, permanent.
  • 550 5.7.25: no reverse DNS for the IP, permanent.
  • 550 5.7.1: refused for a policy reason, permanent.
  • 421 4.7.0: temporary refusal, connection closed, temporary.
  • 550 5.1.1: mailbox does not exist, permanent.

Seeing an API error rather than an SMTP code? Mailbay's own errors, each with a fix, are on the errors page.

Questions

Does 550 5.7.26 mean my domain is blacklisted?

No. It means this message failed authentication. Fix SPF and DKIM for the domain in your From address, add DMARC, and the same mail is accepted again.

Is p=none enough for Gmail?

Google's guidelines accept a DMARC policy of p=none for senders of more than 5,000 messages a day, as long as SPF and DKIM are set up and the From domain aligns with one of them.

Why did forwarded mail get 550 5.7.26?

Forwarding sends the message from the forwarder's server, so SPF no longer matches the original domain. A DKIM signature survives forwarding when the message is not changed, which is why DKIM matters most.

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