In a Microsoft Exchange Server 2013 environment, when you send or forward an S/MIME protected email that contains attachments in Microsoft Outlook in Online Mode, the attachments of the email may be duplicated. Additionally, when you send the S/MIME email 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.
This issue occurs because a property that's related to attachments isn't returned properly by Exchange server 2013 when Outlook is in Online Mode.
To fix this issue, install Cumulative Update 10 for Exchange Server 2013.
To work around this issue, set your Outlook 2013 account to Cached Exchange Mode.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.