4.7.3 Email Bounce Error

Reason

Security conversion required but not possible: Delayed Message

The email message has not yet been delivered because the recipient’s email program and the sender’s email program had issues with the security protocols between them.

How to fix?

Retry the email message see if it was a one-time problem or if the same error occurs. Otherwise, contact the support team as the email system may have a security issue. This is not typically something that you can fix.

4.7.3- 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.7.3

The sender is not authorised to send a message to the intended mailing list. This is useful only as a permanent error.

5.7.3 Email Bounce Error

Reason

Security conversion required but not possible: Message Not Delivered

The message could not be delivered because the recipient’s email program and the sender’s email program had issues with the security protocols between them.

How to fix?

Retry the email message see if it was a one-time problem or if the same error occurs. Otherwise, contact the support team as the email system may have a security issue. This is not typically something that you can fix.

5.7.3- 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.7.3

The sender is not authorised to send a message to the intended mailing list. This is useful only as a permanent error.

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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