Howto: Using EWS, get the original body type of a message without getting the best body of the message.

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The PidTagNativeBody property (Property ID, 0x1016; Data Type 0x0003 (PtypInteger32)) should reflect the format of the body when a normal email message arrives in the inbox (not counting calendar messages such as meeting requests, etc.). If the message is signed, you would need to dig into it to figure out what it is.

2.2.1.22.2 PidTagNativeBody
https://msdn.microsoft.com/en-us/library/ee218029(EXCHG.80).aspx  

2.904 PidTagNativeBody
https://msdn.microsoft.com/en-us/library/ee203274(EXCHG.80).aspx  

Signed messages would require you to pull the MIME to get at the content – clear signed messages are stored as an attachment to the message, however you need to access its contents through parsing MIME content rather than trying to pull that attachment through normal EWS calls.