4.7.8 Email Bounce Error

Reason

User Authentication Failed: Delayed Message

The email message has not yet been delivered because the sender’s email program did not provide your correct username and password to the sender’s email system, which requires these to prove your identity.

How to fix?

In the sender’s email program’s account settings, look for “SMTP” (which is about sending email) and, in the SMTP area, “authentication”. Ensure that the username and password are entered correctly there.

If this error persists, contact the email provider for help.

4.7.8- 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.7.8

This response to the AUTH command indicates that the authentication failed due to invalid or insufficient authentication credentials. In this case, the client SHOULD ask the user to supply new credentials (such as by presenting a password dialog box).

5.7.8 Email Bounce Error

Reason

User Authentication Failed: Message Not Delivered

The message could not be delivered because the sender’s email program did not provide the sender’s correct username and password to the sender’s email provider email system, which requires these to prove the sender’s identity.

How to fix?

In the sender’s email program’s account settings, look for “SMTP” (which is about sending email) and, in the SMTP area, “authentication”. Ensure that the username and password are entered correctly there.

If this error persists, contact the email provider for help.

5.7.8- 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.7.8

This response to the AUTH command indicates that the authentication failed due to invalid or insufficient authentication credentials. In this case, the client SHOULD ask the user to supply new credentials (such as by presenting a password dialog box).

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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