Gmail
Gmail 550 5.7.26 — "This mail is unauthenticated"
550 5.7.26
Gmail rejected the message because it arrived with neither a passing SPF check nor a valid DKIM signature. Since Gmail's 2024 sender requirements, this is a hard requirement for every sender, not just bulk senders — mail with no authentication at all is refused outright rather than spam-foldered.
How to fix it
- 1
Check your SPF record
Run the SPF checker against your domain. No record, a second SPF record on the same domain, or a PermError from too many DNS lookups all read as "no SPF" to Gmail.
- 2
Check DKIM is actually signing
Run the DKIM checker. If your sending platform isn't signing with your domain's key, turn on custom-domain DKIM in that platform's settings — see the setup guides for the exact steps.
- 3
Confirm with a real test
Send a message to the inbox test and confirm both SPF and DKIM pass for your domain specifically, not your email platform's own domain.
- 4
If it's intermittent
Different sending paths (CRM, billing system, marketing tool) often authenticate differently. Publish a DMARC record at p=none with a report address — the aggregate reports list every source sending as your domain and exactly which one is failing.
Meaning current as of 2026-07-14, cross-checked against the provider's own documentation. Providers do occasionally redefine codes — if this doesn't match what you're seeing, the source link is the authoritative reference.