However, you can send other email messages to the same user successfully during the same period.
This problem occurs when the email message is a multipart/mixed MIME message which contains a text/plain body and an application/applefile body. If the corresponding body is not a real application/applefile body, the Exchange Server 2007 server considers that the email message is invalid. Therefore, it generates the NDR. However, Exchange Server 2010 can deliver this kind of email message successfully.
To resolve this problem, install the following update rollup:
2279665 Description of Update Rollup 1 for Exchange Server 2007 Service Pack 3
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about application/applefile, visit the following Microsoft website: