Gmail is refusingyour mail.
550-5.7.26 means Gmail could not authenticate the sending domain and will not take the message. It is a DNS problem, not a content problem.
What is usually wrong
Neither SPF nor DKIM resolves for that domain
Since February 2024 Gmail refuses unauthenticated mail outright. The wording — Unauthenticated email from … is not accepted — is literal: it looked for SPF and DKIM and found neither. Either one passing is enough to stop this particular rejection.
SPF exists but does not include the server actually sending
The record is there, and the IP sending the mail is not in it. This is what happens after you move providers or move just the contact form somewhere else: an include was never added. SPF also fails silently past ten DNS lookups, which is easy to hit with several includes.
DMARC has no aligned identifier
SPF and DKIM can both pass and DMARC still fail, because neither matched the From domain. Any form that sends notifications as the person who submitted it fails here by design. Put your own address in From and theirs in Reply-To.
Shared hosting sends from its own hostname
PHP mail() and the mail features bundled with shared hosting often leave over a shared host name that has nothing to do with your domain's authentication. From says your domain; the authentication does not. To a receiver that is indistinguishable from spoofing.
Two ways out
Fix the DNS
Add the sender to SPF, publish a DKIM key, and start DMARC at p=none while you watch the reports. This is the right long-term answer, and it takes days rather than minutes once propagation and verification are counted — plus access to the DNS.
Send over an authenticated domain instead
For contact form mail specifically, you can hand the sending to infrastructure that is already authenticated. Your domain's records stay untouched. From is the platform, Reply-To is you, so replies still land in your mailbox.
Start freeInstalling takes one line, and pointing an existing form's action at the endpoint works too. Every submission comes back sorted, with a draft reply ready for you to approve.
<script src="https://nintact.com/embed.js"
data-nintact="YOUR_FORM_ID" async></script>Questions
What does 550-5.7.26 mean exactly?
Gmail is saying it cannot authenticate the sending domain, so it will not accept the message. It is returned when neither SPF nor DKIM passes for that domain. If either one passes, you do not see this error.
Is SPF alone enough?
It stops this rejection. To stay out of the spam folder reliably you also want DKIM, and bulk senders are expected to publish DMARC as well.
Can I fix this without editing DNS?
For form mail, yes. If the message leaves over an already-authenticated sending domain, nothing about your own domain has to change. nintact works that way — setup is confirming an email address.
How do I see where my domain stands right now?
Put it into the deliverability check. It resolves SPF, DKIM, DMARC and MX live and tells you whether mail from it reaches Gmail today. No account needed.