4.3.3 Email Bounce Error

Reason

Message Features Not Supported By Receiving System: Delayed Message

The email message has not yet been delivered because the message seems to contain features not supported by the receiving system. Typically, this occurs when systems have differing capabilities.

How to fix?

Contact the sender’s email system support team to investigate what feature of your email system may not be supported by the receiving system. This usually requires some level of support to fix.

4.3.3 – Official definition from ITEF

For the code that starts from 4.X.X

A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful.

For the code that starts from X.3.3

Selected features specified for the message are not supported by the destination system. This can occur in gateways when features from one domain cannot be mapped onto the supported feature in another.

5.3.3 Email Bounce Error

Reason

Message Features Not Supported: Message Not Delivered

The message could not be delivered because the message you are sending seems to contain features not supported by the receiving system. Typically, this occurs when systems have differing capabilities.

How to fix?

Contact the sender’s email system support team to investigate what feature of your email system may not be supported by the receiving system. This usually requires some level of support to fix.

5.3.3- Official definition from ITEF

For the code that starts from 5.X.X

A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.

For the code that starts from X.3.3

Selected features specified for the message are not supported by the destination system. This can occur in gateways when features from one domain cannot be mapped onto the supported feature in another.

Pankaj Kumar is an Email Deliverability Expert and Product Leader with 13 years of experience in email deliverability, CRM, marketing analytics, deliverability onboarding, implementation, and automation. He combines deep technical expertise in email infrastructure, inbox placement, sender reputation, and authentication protocols with a product-driven approach to solving complex deliverability challenges at scale. Having worked with Zeta and Blueshift, Pankaj has helped hundreds of clients improve email performance through data-driven strategies, technical troubleshooting, and scalable automation. His focus extends beyond resolving deliverability issues to building smarter deliverability products, monitoring systems, and AI-powered solutions that help businesses proactively identify risks and improve inbox placement. Through his blog at University of Email, Pankaj shares practical insights on email deliverability, CRM strategy, marketing analytics, and emerging deliverability technologies to help email marketers and technical professionals make informed decisions and achieve better results. Connect with him on LinkedIn at https://www.linkedin.com/in/pankajkumar-deliverability

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