What the rejection tells you
A Gmail 550-5.7.26 response means the receiving system rejected the message because required authentication or sender controls were not satisfied. Read the complete enhanced-status text in the bounce: it often names SPF, DKIM, DMARC, or unauthenticated mail rather than pointing to one universal fix.
Test the domain in the visible From address, but also identify the envelope sender, DKIM signing domain, and outbound IP used by the failed message. Testing a corporate domain while the application sends through a different return-path can hide the real problem.
Repair authentication in order
Confirm one valid SPF record authorizes the sending service. Enable DKIM with the provider's current selector. Publish DMARC and make sure either SPF or DKIM aligns with the visible From domain. For direct mail servers, also verify forward and reverse DNS and use TLS.
A DNS checker cannot measure spam complaint rate or confirm one-click unsubscribe headers. Bulk senders should review those controls in the platform and monitor Postmaster Tools where available.
Retest the failed mail stream
Wait for DNS propagation, send a new message through the same application, and inspect its Authentication-Results. Do not rely on a test from a different provider. If the rejection remains, compare the new bounce text and sending IP with the repaired configuration.