Cannot view opaque-signed message when using non-S/MIME clients

Problem description

As discussed in Understanding Message Security, a digitally signed message can either be clear signed or opaque signed. The sender's e-mail client makes the determination of how a message is signed. For example, in Microsoft Outlook®, on the Security tab of the Options dialog box, the user can select the Send clear signed messages when sending signed messages check box to specify if the messages will be clear signed.

If a message is sent as an opaque-signed message and that message is viewed in an e-mail client that does not support S/MIME functionality, that e-mail client will not be able to display the message. Instead, the message will display as a blank message with an attached .p7m file that is unreadable. Note that the same message will display successfully in an e-mail client that does support S/MIME.

Resolution

To address this situation, either view the unreadable message in an e-mail client that provides support for S/MIME or request that the sender resend the message either with a cleartext signature or no signature, which will then be viewable in a non-S/MIME e-mail client.

This issue only affects the ability to display the signed message. It is not related to the validation or validity of the signature itself.