Ho to determine if mailitem is encrypted in Outlook 365?

Question

I am writing a VSTO Addin for Outlook 365.
I need to be able to determine in code if an email (MailItem) is encrypted.
I've tried examining the following properties - both indicate that the email is not encrypted but it clearly is:

mailItem.MessageClass -> "IPM.Note"  
mailItem.PropertyAccessor.GetProperty(PR_SECURITY_FLAGS) -> 0  

What am I doing wrong?

Answer 1

Thanks for your efforts, Timon.
I am sure there is an answer somewhere out there...

I've looked at everything there is in the MailItem object model using OutlookSpy

The only properties that I found to be of any use are these two:

On the encrypted email:

mailItem.PropertyAccessor.GetProperty(PR_ICON_INDEX) -> 306  
mailItem.PropertyAccessor.GetProperty(PR_BODY_W) -> "This message is protected with Microsoft Information Protection..."  

Answer 2

Thanks for the insights on encryption. All the solutions I could find on checking encryption failed for me as well. Your solution of looking for the PR_ICON_INDEX worked, however I have found two values relating to encryption: 306 and 308. I will add them here if I detect anymore.

Belt and braces(!) I also wrote my code to support the proptag http://schemas.microsoft.com/mapi/proptag/0x6E010003 in case I encounter any mails with this set.

Function isEncrypted(inboxItem As Object) As Boolean

    ' Code from https://learn.microsoft.com/en-us/answers/questions/484515/ho-to-determine-if-mailitem-is-encrypted-in-outloo

    Dim PropAccessor As Outlook.PropertyAccessor
    On Error GoTo isEncryptedError
    
    isEncrypted = False

    Set PropAccessor = inboxItem.PropertyAccessor
    If (PropAccessor.GetProperty("http://schemas.microsoft.com/mapi/proptag/0x10800003") = 306 Or _
        PropAccessor.GetProperty("http://schemas.microsoft.com/mapi/proptag/0x10800003") = 308) Then '
            isEncrypted = True
    End If

    If PropAccessor.GetProperty("http://schemas.microsoft.com/mapi/proptag/0x6E010003") And 1 <> 0 Then   '
        isEncrypted = True
    End If

isEncryptedExit:
    Set PropAccessor = Nothing
    Exit Function

isEncryptedError:
    isEncrypted = False
    Resume isEncryptedExit
               
End Function

Answer 3

I checked many documents of Microsoft.Office.Interop.Outlook, and tried other packages (EWS), but unfortunately, I didn't find any package that has a property to indicate whether this email is encrypted or not.

But when I tried it, I found that we can use its HtmlBody to determine whether the current email is encrypted. Normal emails show normal content, while encrypted emails look like this:

We can check whether the body contains specific text. This method may not be very elegant, but it should work.

You decide whether to use it.

Part of the code I used:

            ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2013_SP1);  
            service.Credentials = new WebCredentials("test@outlook.com", "password");  
            service.Url = new Uri("https://outlook.office365.com/EWS/Exchange.asmx");  
  
            Mailbox mb = new Mailbox("test@outlook.com");  
            FolderId fid = new FolderId(WellKnownFolderName.Inbox, mb);  
            FindItemsResults<Item> findResults;  
            ItemView view = new ItemView(10);  
  
            findResults = service.FindItems(WellKnownFolderName.Inbox, view);  
            foreach (var item in findResults.Items)  
            {  
                item.Load();  
                Console.WriteLine(item.Subject);  
                Console.WriteLine(item.Body);  
                Console.WriteLine("~~~~~~~~~~~~~~~~~~~~~~~~~");  
            }  
            Console.Read();  

Update(7/23):

You can also use Microsoft.Office.Interop.Outlook to get the HtmlBody of the email.

It seems that this library also doesn't provide a ready-made property for identifying whether the email is encrypted, so I can only do it in the same way, just switch the library used.

            Application outlookApplication = new Application();  
            NameSpace outlookNamespace = outlookApplication.GetNamespace("MAPI");  
  
            MAPIFolder theInbox = outlookNamespace.Folders[1];  
            MAPIFolder re = theInbox.Folders["InBox"];  
            Items mailItems = re.Items;  
              
            var mail = mailItems.GetFirst();  

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