Status information about the SMTP message which is being sent.
Amount of data from the message already sent to the server in bytes.
Note that the SMTP MTM encodes the text and binary data inside an email message while it is sending the data. As a result, iBytesToSend is likely to increase (by as much as one third) while a message is being sent. The size increase represents all additional bytes which have been added to the RFC 822 message to encode the data.
Amount of data from the message still to be sent to the server in bytes.