One of the new error messages that Google may return for Gmail recipients is:
550, “5.7.26”, “This message does not have authentication information or fails to pass authentication checks (SPF or DKIM). To best protect our users from spam, the message has been blocked. Please visit Prevent mail to Gmail users from being blocked or sent to spam for more information.”
This new error message is a permanent failure for senders who do not have a valid SPF or DKIM record.
To resolve this and get email delivered to Gmail again, you must implement DKIM or SPF, ideally both.