rfc822Message, which represents a
collection of ``email headers'' as defined by the Internet standard
RFC 822. It is used in various contexts, usually to read such headers
from a file.
A Message instance is instantiated with an open file object as
parameter. Instantiation reads headers from the file up to a blank
line and stores them in the instance; after instantiation, the file is
positioned directly after the blank line that terminates the headers.
Input lines as read from the file may either be terminated by CR-LF or by a single linefeed; a terminating CR-LF is replaced by a single linefeed before the line is stored.
All header matching is done independent of upper or lower case;
e.g. m['From'], m['from'] and m['FROM'] all yield
the same result.