I need to find out, whether Exchange EmailMessage is marked as Do not forward. Which property should I look for?
Here is documentation of class EmailMessage.
This flag can be seet up in Outlook for example:
If you work with email messages in transport pipeline, you find this information in superior MailItem object. In properties you will find property Microsoft.Exchange.RightsManagement.DecryptionTokenRecipient indicating which user has permission to decrypt the message.
If you work with EWS look at MailItem and its extended properties (not tested).