The stream must contain data in MIME format, other formats are not supported. MIME files often have the '.eml' extension and are produced by Outlook Express. In contrast, Outlook produces '.msg' files that are not MIME files and therefore cannot be sent using this method.
Both sender and the list of recipients are extracted from the message. The sender address is taken from the "Sender" header field, or from the first address in the "From" field if the "Sender" is missing. The list of recipients comes from "To", "Cc" and "Bcc" header fields. Also, the "Bcc" field is removed from message headers prior to sending.
Set the AllowNullSender property to allow sending a mail without a sender specified.