PR_INTERNET_MESSAGE_ID returns blank when sending message

问题

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();

回答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?

来源：https://stackoverflow.com/questions/19029082/pr-internet-message-id-returns-blank-when-sending-message