ADDRESS_HEADERS is a set containing strings: 'from', 'sender', 'reply-to', 'to', 'cc', 'bcc', 'resent-from', 'resent-sender',
ADDRESS_HEADERS = set ( [ 'from' , 'sender' , 'reply-to' , 'to' , 'cc' , 'bcc' , 'resent-from' , 'resent-sender' , 'resent-to' , 'resent-cc' , 'resent-bcc' , ] )