The following message could not be forwarded to email@example.com
Technical details of the failure:
This message does not have authentication information or fails to
pass authentication checks. To best protect our users from spam, the
message has been blocked. Please visit
information. np9-20020a17090b4c4900b001dd67f12d27si173372pjb.106 - gsmtp
Many other mails are forwarded without any problem but one of them is blocked. What can I do in order to consider this email as safe ?