PR_INTERNET_MESSAGE_ID returns blank when sending message

Question

I'm developing add-in for Outlook 2013,I'm struggling with PR_INTERNET_MESSAGE_ID , I could easily get PR_INTERNET_MESSAGE_ID on archiving incoming mails, but it returns nothing on sending mail. Any help would be appreciated! Thank you! here is my code

 string PR_INTERNET_MESSAGE_ID = "http://schemas.microsoft.com/mapi/proptag/0x1035001F";
        Microsoft.Office.Interop.Outlook.PropertyAccessor pal = mailItem.PropertyAccessor;
        string Internet_Message_Id = pal.GetProperty(PR_INTERNET_MESSAGE_ID).ToString();

Answer 1

That property is only sent after the message is sent. Exchange 2010/2013 sets that property for all messages created in the store, but you will not necessarily see it on messages in the cached store. Are you using PST or Exchange?