Other headers as determined by the user (optional).
This property can be set to a string of headers to be appended to the HTTP request headers created from other properties like ContentType, From, etc.
The headers must be of the format "header: value" as described in the HTTP specifications. Header lines should be separated by CRLF ("\r\n") .
Use this property with caution. If this property contains invalid headers, HTTP requests may fail.
This property is useful for extending the functionality of the class beyond what is provided.
This property is not available at design time.