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.