Gmail "550-5.7.26 unauthenticated email" rejections

Gmail's bounce is explicit about the requirement:

550-5.7.26 This mail has been blocked because the sender is
550-5.7.26 unauthenticated. Gmail requires all senders to
550-5.7.26 authenticate with either SPF or DKIM.

Since February 2024 Gmail refuses mail that authenticates with neither SPF nor DKIM. Senders above ~5,000 messages/day face the stricter bulk-sender rules: both SPF and DKIM, a DMARC record, alignment with the From: domain, one-click unsubscribe for marketing mail, and spam-rate under 0.3% in Postmaster Tools.

Why it happens

Typical culprits: a web server or application sending directly with no DKIM signature and an SPF that does not include it; a forwarder breaking SPF (envelope rewriting) with no DKIM to fall back on; or DNS changes that dropped a selector record. IPv6 senders are hit hardest — Gmail hard-requires authentication on IPv6 connections.

Diagnose it

The Google/Yahoo compliance check below grades every one of these requirements for your domain in one pass, and links each failing item to the specific record to fix. Follow up with the DKIM tool if you are unsure which selectors exist.

Fix it

1. Ensure the actual sending path signs with DKIM for your domain (application mail should relay through a signing service).

2. Cover every sender in one SPF record ending ~all.

3. Publish DMARC (p=none to start) — required for bulk senders and it makes alignment visible via reports.

4. For bulk mail: add List-Unsubscribe headers and watch your spam rate in Google Postmaster Tools.