Write an OnPostingData event handler to provide the end-user with feedback of the progress when sending a request that contains a large amount of data. When the content of a request is very large (for example, when it contains large attachments), the Send method breaks the message into blocks, which it sends separately. After sending each of these blocks, it generates an OnPostingData event. You can use the event handler to update a progress bar or similar control to indicate how much of the message has been sent. 

OnBeforePost 

Send 

OnReceivingData