There are several possible causes for this code. One of the most common ones is that the mail account does not exist at the ...

{4}: There are several possible causes for this code. One of the most common ones is that the mail account does not exist at the organization the message was sent to. This issue may occur if there was a problem when users were moved between sites. For example, if a user is moved to a server in a different administrative group and then replies to an old message, or if the user does not recreate his or her Outlook profile, the old administrative group legacy address is used, and a non-delivery report is generated. Troubleshooting: Verify that the recipient address is formatted correctly and resend the message.