5.6.10

Message Not Delivered: email servers are not responding appropriately

The message could not be delivered because the sending and receiving email systems had an issue processing the email. Likely, there was an issue with the format of the email addresses involved in the message.

Fixes

Check the sender's email address or the recipient's email address. Look for addresses that may have special characters for letters. If any are found, try correcting and retry the message.

Otherwise, contact the sender's email support team to see if they can help troubleshoot further.

5.6.10: Official Definition

5.X.X

[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 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.

X.6.10

[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.6.10:

This indicates that a reply containing a UTF-8 string is required to show the mailbox name, but that form of response is not permitted by the SMTP client.