There is not a good way to mark a MessageType
as read using EWS v1. See this MSDN blog post for a workaround if you are stuck with EWS v1.
EWS v2 introduced the IsRead property as writable (message.IsRead = true; message.Update();
), which makes it trivial. You can use the EWS v2 managed API for Exchange 2007 and above, but it is a separate installation. See the MSDN page on EWS Managed API 2.0 for more.