Emails with digital signatures don't work on KanBo Outlook App
KanBo Outlook App works good with common emails. The KanBo Outlook App doesn't launch for emails with digital signatures (emails signed with the sign below).
This is an answer to your problem, provided by Microsoft:
So, I hope the fact is confirmed and you are right, add-ins are not working with those types mail items. The reason the add-ins are not working with those types of items is very simple. Those items are signed by the sender and if Microsoft allow to manipulate with parts of those messages the signature will be invalidated and verification will fail. Those are the message which you cannot temper with. You may say that you want to use the message in kinda "read only" mode, so Office.JS API will work to get info, but never set anything back. It would be reasonable request which you may submit to Office Developer Platform UserVoice page
And finally, I strongly believe, instead of allow "read only" access to those messages, Microsoft just restricted access to them completely. Do they have any plans to implement something like this you probably should ask through User Voice website as well. As the "partner" you should have access to submit internal request for the feature if you need as well.
Source of the text: https://social.msdn.microsoft.com/Forums/office/en-US/89ec70df-4c98-4ce8-aba2-08c8191d8f27/outlook-addins-on-emails-with-digital-signatures?forum=appsforoffice