How to check if outlook email encrypted?

Question

I use Outlook 2016 from Office 2016. I develop VSTO Outlook plugin which upload email to the database (initially it save it as temp file on the hard drive). How to check if email is encrypted? How to check if email Signed? See first image which show how user encrypt it.

Note: when recieve and decrypte email, it appears "normal", not decrypted. But other users can't open saved email, see second image with message "Your Digital ID name cannot be found by the underlying security system." Currently there is not task to decrypt emails, only not to add encrypted in the system. Thank you.

if (item is Outlook.MailItem)
{
    var mailItem = item as Outlook.MailItem;
    // Check
    //if (mailItem.IsEncrypted) ...   // Here I need help
    //   show user warning and exit 
    mailItem.SaveAs(tmpEmailFile);
    ...

Answer 1

Use the PR_SECURITY_FLAGS property which is available in Extended MAPI (a low-level API on which Outlook is based on). The PropertyAccessor.GetProperty can help with retrieving the property value. The DASL name of the property is "http://schemas.microsoft.com/mapi/proptag/0x6E010003". The value can be one of the following constants:

Value	Const	Description
NONE	0x0000	Message has no security
ENCRYPTED	0x0001	Message is encrypted
SIGNED	0x0002	Message is signed
SIGNED & ENCRYPTED	0x0003	Message is signed and encrypted

See Outlook 2016 VB.NET Sign and Encrypt Message for more information.