When a Microsoft Exchange Server 2007-based Edge server receives signed e-mail messages that have an attachment from the Internet, the Microsoft Outlook MAPI users receive the messages. However, the digital signature is invalid. If you click the signature icon in Microsoft Outlook, you receive the following error message:
The digital signature is invalid
This problem occurs because the Attachment Filter agent adds the Content-Description field together with the file name of the attachment to the message. For the messages that are signed, this modification invalidates the digital signature.
To resolve this problem, install Update Rollup 2 for Exchange Server 2007 Service Pack 1 (SP1) on the Exchange server. For more information about Update Rollup 2 for Exchange Server 2007 Service Pack 1, see the following Exchange Help topic: