4.6.4 Email Bounce Error

Reason

Content Loss During Delivery: Delayed Message

The email message has not yet been delivered because it included content or attachments that needed to be converted and information was lost in the process.

How to fix?

Contact the recipient(s) to ensure that no undesirable consequences result from information being lost from this message before it was delivered.

If the message had attachments, try sending it again without them, and then try sending each attachment in a separate email. If this error occurs when there are no attachments, check the text of the message for unusual content or characters. If it occurs with a specific type of attachment, consider sending that information in another format.

4.6.4- 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.6.4

This is a warning sent to the sender when message delivery was successfully but when the delivery required a conversion in which some data was lost. This may also be a permanent error if the sender has indicated that conversion with loss is prohibited for the message.

5.6.4 Email Bounce Error

Reason

Content Loss During Delivery: Message Not Delivered

The message could not be delivered because it included content or attachments that needed to be converted and information was lost in the process.

How to fix?

Contact the recipient(s) to ensure that no undesirable consequences result from information being lost from this message before it was delivered.

If the message had attachments, try sending it again without them, and then try sending each attachment in a separate email. If this error occurs when there are no attachments, check the text of the message for unusual content or characters. If it occurs with a specific type of attachment, consider sending that information in another format.

5.6.4- 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.6.4

This is a warning sent to the sender when message delivery was successfully but when the delivery required a conversion in which some data was lost. This may also be a permanent error if the sender has indicated that conversion with loss is prohibited for the message.

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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