Symptoms
In a Microsoft Exchange Server 2013 environment, when you send or forward an S/MIME-protected email message that contains attachments in Microsoft Outlook in Online Mode, the message attachments may be duplicated. Additionally, when you send the S/MIME email message to external recipients, you may receive a non-delivery report (NDR) that contains the following error message:
Remote Server returned '550 5.6.0 M2MCVT.StorageError.Exception: ConversionFailedException, Content conversion: Invalid S/MIME encrypted message.; storage error in content conversion.
Cause
This issue occurs because a property that's related to attachments isn't returned correctly by Exchange Server 2013 when Outlook is in Online Mode.
Resolution
To fix this issue, install Cumulative Update 10 for Exchange Server 2013.
Workaround
To work around this issue, set your Outlook 2013 account to Cached Exchange Mode.
Status
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
