Gmail 5.7.26 errors usually point to sender authentication or policy problems. Start by checking whether the envelope sender, header From domain, SPF, DKIM, and DMARC are working together instead of treating the error as a generic bounce.
Run the compliance checker for the domain in the visible From address. If SPF or DMARC is missing, publish those records first. If DKIM is missing, copy the selector from the email provider and test selector._domainkey.yourdomain.com.
After DNS looks correct, send a fresh test message and inspect the raw headers. You want SPF or DKIM to pass, and you want DMARC to pass through alignment with the visible From domain.