The message structure, after initiating a parse() cycle, looks like this:
Message object, also top-level node in Message::Node tree | +---> Message::Node for other parts in MIME structure | |---> [ more Message::Node parts ... ] | [ others ... ] | +---> Message::Metadata object to hold metadata
"message" is either undef (which will use STDIN), a scalar of the entire message, an array reference of the message with 1 line per array element, and either a file glob or IO::File object which holds the entire contents of the message.
Note: The message is expected to generally be in RFC 2822 format, optionally including an mbox message separator line (the ``From '' line) as the first line.
"parse_now" specifies whether or not to create the MIME tree at object-creation time or later as necessary.
The parse_now option, by default, is set to false (0). This allows SpamAssassin to not have to generate the tree of Mail::SpamAssassin::Message::Node objects and their related data if the tree is not going to be used. This is handy, for instance, when running "spamassassin -d", which only needs the pristine header and body which is always handled when the object is created.
"subparse" specifies how many MIME recursion levels should be parsed. Defaults to 20.
If called in an array context, an array will be returned with each specific header in a different element. In a scalar context, the last specific header is returned.
ie: If 'Subject' is specified as the header, and there are 2 Subject headers in a message, the last/bottom one in the message is returned in scalar context or both are returned in array context.
Note: the returned header will include the ending newline and any embedded whitespace folding.
For multipart messages, the first call to parse_body() doesn't create a new sub-tree and just uses the parent node to contain children. All other calls to parse_body() will cause a new sub-tree root to be created and children will exist underneath that root. (this is just so the tree doesn't have a root node which points at the actual root node ...)