[2.1] Email Message SCO is valid with only the is_multipart property?

[sr@ctin.us]

Fwd from: https://github.com/oasis-tcs/cti-stix2/issues/182

See follow up comments in the issue from community members for further context


[2.1] Email Message SCO is valid with only the is_multipart property?

In section 6.6 all properties are optional except for is_multipart.

The body and body_multipart fields have rules around usage based on  
the is_multipart. But the rule is "this property MUST NOT be used if  
is_multipart is true/false"

Should this rule be updated to state that a body or body_multipart  
must be provided? Or is a email message technically valid with just  
the common props and the is_multipart prop?