@Seb T,
This is a tough one. I think the only way around this is to change the response content handling in the service definition and remove the return collection processing. Then, within your process/VBO you could process the raw
Response Content data item yourself with Newtonsoft and handle whatever exceptions are raised by it directly. I've seen a few examples online of people creating their own extensions of the JSON serialization code, but none of them show examples of super complex structures like what Outlook presents for a message.
If you're organization uses a standard Outlook message template definition you could even define a .NET class to implement that template and then use Newtonsoft to serialize the message directly to an instance of that class type.
Cheers,