The content type of the outgoing message.
This property is used to specify the content type (MIME type) for outgoing messages sent using SendMessage. The default value is text/plain, which should be sufficient is most cases.
Note that this property is ignored for messages sent using SendData, as such messages do not have a content-type header added to them. The absence of a content-type header indicates to the server that the data should be treated as a binary blob.
This property is not available at design time.