Common Email Delivery Error Codes
Delivery summary: Any time you see, “This message was rejected by its destination for reasons outside the control of Sentinel Messaging, and got returned to the sender”, this literally means that this is outside our control, and this issue must be with the Customer/Recipient server accepting the message.
452 4.3.1 Insufficient system resources: This normally means that the recipient/customers server doesn’t have enough resources to accept messages. Customer/recipient needs to resolve this issue before we can deliver the message.
550 5.7.1 : Recipient address rejected: User email address is marked as invalid. This means normally you have SMTP Discovery enabled and the user is on the SMTP Discovery Invalid list. Remove this user from this list to resolve this issue.
550 5.1.0 sender rejected: The senders IP address is rejected due to a Blacklist/ wrong SPF. This is outside the control of Sentinel Messaging.
550 5.7.1 Relay Access denied: We are a closed relay system. This means if a user is in the Sentinel Messaging system we will deny access to any e-mail that is not registered.
For more information and understand on error codes please visit https://tools.ietf.org/html/rfc3463