Adds a header to the raw SMTP message.
Syntax:
Parameters:
Returns:
Returns 1 if successful; otherwise returns 0 if the operation fails.
Remarks:
If the header does not exist then the header is added to the message. Otherwise, if the header already exists then the value of the existing header is overwritten by the new value specified in sValue.
A case insensitive comparison is made when looking for an existing header.