How to set the encrypted Flag for an Outlook MailItem?

I need to be able to programmatically send encrypted password Emails with an Excel VSTO Add-In so i have written this static method to send Outlook Emails:

private static Outlook.Application outlookApp;
private const string PR_SECURITY_FLAGS = "http://schemas.microsoft.com/mapi/proptag/0x6E010003";

public static void SendPasswordEmail(string emailAddress, string password, string subject, bool PGP)
		{
			GetOutlookApp();
			Outlook.MailItem eMail = (Outlook.MailItem)outlookApp.CreateItem(Outlook.OlItemType.olMailItem);
			// Check if the Email should be prepared to be sent with PGP or PKI encryption.
			if (PGP)
			{
				eMail.Subject = String.Concat("PGP: ", subject);
			}
			else
			{
				eMail.Subject = subject;
				// Set the Security Flags of the current MailItem to encrypted.
                // already tried 1,2
				eMail.PropertyAccessor.SetProperty("PR_SECURITY_FLAGS", 1);
			}
			String eMailBody = String.Format(@Properties.Resources.PasswordEmailBody, password);
			eMail.To = emailAddress;
			eMail.BodyFormat = Outlook.OlBodyFormat.olFormatHTML;
			eMail.HTMLBody = eMailBody;
			eMail.Send();
		}

i am using this method to get the Outlook Application:

private static void GetOutlookApp()
		{
			// Check whether there is an Outlook process running.
			if (Process.GetProcessesByName("OUTLOOK").Count() > 0)
			{
				// If so, use the GetActiveObject method to obtain the process and cast it to an Application object.
				outlookApp = Marshal.GetActiveObject("Outlook.Application") as Outlook.Application;
			}
			else
			{
				// If not, create a new instance of Outlook and sign in to the default profile.
				outlookApp = new Outlook.Application();
				Outlook.NameSpace session = outlookApp.GetNamespace("MAPI");
				session.Logon("", "", false, true);
			}
		}

But i cant seem to set the security flag of the MailItem to encrypted. No Exception thrown ! it just that outlook doesn't send the Email as encrypted !

i have this VBA Code which is working fine:

Const PR_SECURITY_FLAGS = "http://schemas.microsoft.com/mapi/proptag/0x6E010003"
Dim prop As Long

Set MailItem = oApp.CreateItem(olMailItem)
MailItem.To = something
MailItem.Subject = something
MailItem.BodyFormat = olFormatPlain
MailItem.Body = something

// set encrypted flag
Set MailItem = Application.ActiveInspector.CurrentItem
prop = CLng(MailItem.PropertyAccessor.GETPROPERTY(PR_SECURITY_FLAGS))
ulFlags = ulFlags Or &H1 ' SECFLAG_ENCRYPTED
MailItem.PropertyAccessor.SetProperty PR_SECURITY_FLAGS, ulFlags
MailItem.SEND

can you please explain to me what is this code block doing exactly?

Set MailItem = Application.ActiveInspector.CurrentItem
prop = CLng(MailItem.PropertyAccessor.GETPROPERTY(PR_SECURITY_FLAGS))
ulFlags = ulFlags Or &H1 ' SECFLAG_ENCRYPTED
MailItem.PropertyAccessor.SetProperty PR_SECURITY_FLAGS, ulFlags

Is there a refrence or a List of the available MAPI properties and their accepted values ?

You must use a DASL name of the PR_SECURITY_FLAGS property, not a property name. For example:

private const string PR_SECURITY_FLAGS = "http://schemas.microsoft.com/mapi/proptag/0x6E010003";
eMail.PropertyAccessor.SetProperty(PR_SECURITY_FLAGS, 1);

See How to sign or encrypt a message programmatically from OOM for more information.