Expecting our secure message receipts to behave exactly like Outlook message receipts is just plain silly. Here’s a tip: our application is NOT OUTLOOK. No, receipts returned by our mail encryption system do not use Outlook-specific properties like "OutlookMessageClass". Since our receipt is just an email message, it’s up to Outlook to decide what message class it is. If it doesn’t set it to the same "class" as the return receipts generated BY Outlook, well, we have no control over that.
(Tip number 2: Yes, Outlook/Exchange dominate the business email market. However they do NOT define how email works. Please stop expecting everything on the Internet to conform to the Microsoft Way.)