XADM: Digitally Signed Messages Are Received Without the Signature

When you send a message that has a digital signature, the message may be received without the signature.

In Exchange Server 5.5, the Internet Mail Service does not deliver signed messages by default. Messages that are sent with a digital signature are modified by the Internet Mail Service and delivered without the signature.

To resolve this issue, change the Internet Mail Service to allow S/MIME signatures:

1. Start the Exchange Server Administrator program, and go to the Connectors container.
2. Double-click Internet Mail Service.
3. Click the Internet Mail tab.
4. Click to select the Clients support S/MIME signatures check box.
5. Restart the Internet Mail Service.

Client-Side Workaround

If you are sending messages from a client computer, you can encode the signature and message to prevent the signature from being removed:

1. Start Outlook Express, and on the Tools menu, click Options.
2. Click the Security tab and click the Advanced button.
3. Click to select the Encode message before signing (opaque signing) check box.

When you use this option to encode the message, the process is similar to encrypting the message. However, this option does not require the receiver to have a certificate to decode the message. An S/MIME-aware client can decode and view the message. Non-S/MIME-aware clients receive the message in an attached file, Smime.p7m.

The mail icon for a message that is sent with a digital signature is marked with a red ribbon. If you save a copy of the message in the Sent Items folder, you can check if the message that was sent has a digital signature by checking to see whether the message is marked with a red ribbon.

The following information about the Clients support S/MIME signatures option can be found in the Exchange Administrator program Online Help: