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.
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.
Here is an example of what to add to the request to get the PidTagNativeBody property:
<t:ExtendedFieldURI PropertyTag="4118" PropertyType="Integer" />