- 421-4.7.0 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.
Remote Server returned '<ASPMX.L.GOOGLE.com #5.0.0 smtp; 550-5.7.1 This message does not have authentication information or fails to pass 550-5.7.1 authentication checks. To best protect our users from spam, the 550-5.7.1 message has been blocked. Please visit 550-5.7.1 https://support.google.com/mail/answer/81126#authentication for more 550 5.7.1 information. 66si555989qte.231 - gsmtp>'
For further information about Gmail's policies for senders, see Gmail Bulk Sender Guidelines.
Reflexion - Solution
- Google has started using a much more strict SPF filtering policy which requires a sending domain's SPF record to be correctly formatted.
Symantec - Solution
- Emails sent to Gmail addresses using the Symantec Email Security.cloud service temporarily fail authentication checks which can cause email delivery to be retried.
Plesk - Solution
- Messages are stuck in mail queue and the following error message appears in