If IMessage::SubmitError fails and returns MAPIEEXTENDED_ERROR, IMessage::GetLastError gets called. If you supply MAPIERROR structure to Outlook it does not use the information on MAPIERROR in the displayed error box. I think the information returned by GetLastError should be shown in the error message popup.
See: https://social.msdn.microsoft.com/Forums/office/en-US/96ca73d6-e4d4-462e-9791-d3e035b288ff/mapi-extended-errors-for-imessage?forum=outlookdev#8803715e-ac2f-4788-b2ad-74eca08b8723