550 5.7.26 error: unauthenticated email (Gmail)
Gmail's 550 5.7.26 means your message failed authentication: SPF and DKIM both failed, or your domain's DMARC policy said to reject it. Fix your DNS records.
What 550 5.7.26 means
550 5.7.26 This email has been blocked because the sender is unauthenticated.Gmail's versions of this code point to three causes: the message failed both SPF and DKIM; your SPF record ends in -all and the sending IP isn't in it; or your DMARC policy told Gmail to reject unauthenticated mail.
List problem or sender problem?
Sender problem. It's your domain's authentication: Gmail couldn't confirm the message really came from you. It's a permanent failure: the server won't accept this message if you send it again unchanged.
How to fix 550 5.7.26
- Set up SPF and DKIM for the sending domain, following Google's sender guidelines.
- If your SPF record ends in -all, make sure every service that sends as your domain is in it.
- If the bounce mentions DMARC, check alignment: SPF or DKIM has to pass for the same domain as your From address.
- Send a test to a Gmail inbox and check the headers show spf=pass and dkim=pass before you resume.
In SendingOS, 550 5.7.26 shows on the conversation with its reason and code. If bounces like this pile up on one domain, a domain health rule can pause that domain's inboxes before the whole campaign suffers.
Not sure whether a spike in bounces is the list or the domain? Read High email bounce rate? Burned domain or bad list.
Related codes
Sources: Google: Gmail SMTP errors and codes
Questions
Is 550 5.7.26 about my email's content?
No. It's about authentication only: Gmail couldn't verify the message came from your domain. Fix SPF, DKIM and DMARC; the copy doesn't matter here.