With SMSMSE enabled, some attachments sent via iPhone or iPad are treated as unscannable. The reason for the unscannable changes from instance to instance, and may include "Decomposer failed to open the container" or "Malformed Container".
In addition, many of these items also contain an attachment with a name similar to "ATT00001".
This is a known issue in Exchange 2007/2010 with certain types of MIME encoded messages. This issue occurs when an attachment body part in an e-mail message is sent before the message body part of the e-mail message is sent.
For additional details, see Microsoft article: "The body of a message is shown incorrectly as an attachment if you try to use an application in an Exchange Server environment to send a message that includes attachments".
This issue is not caused or triggered by SMSMSE, the interaction between the iOS device and the Exchange transport service creates a corrupt attachment, and SMSMSE is simply reacting to this corruption.
There is no solution available for Exchange at this time.
Configure the iOS device to send attachments in a way that Exchange will not seperate out attachment content from the original attachment. See Apple article "Mail (Mountain Lion): Add attachments" for details on changing attachment settings in Apple Mail.
The relevant settings for this issue are:
Once those settings are enabled, the attachments should now be delivered intact. Keep in mind that Apple Mail still allows you to drag and drop attachments - if doing this, be sure to drop them at the end of the message. Attachments dropped in the middle of the message will cause any text that follows the attachment to be converted into another attachment, which will trigger this symptom.