550 5.7.26 — unauthenticated email from domain is not accepted

Gmail rejected the message because it failed both SPF and DKIM alignment. This is a hard bounce, not a spam-folder placement.

What causes it

• A third-party platform sending as your domain with no authentication configured.
• SPF passing for the platform bounce domain but not aligning with your From: domain, and no DKIM signature to fall back on.

How to fix it

Authenticate the specific platform that generated the bounce. The bounce message names the sending IP — match it against your DMARC aggregate reports to identify which service it is, then add its DKIM records and its SPF include.

Check your domain

Also searched for: 550 5.7.26, gmail 5.7.26, unauthenticated email not accepted.