Access a classic Unix-style mailbox, where all messages are contained
in a single file and separated by ``From name time'' lines.
The file object fp points to the mailbox file.
Access an MMDF-style mailbox, where all messages are contained
in a single file and separated by lines consisting of 4 control-A
characters. The file object fp points to the mailbox file.
Access a Babyl mailbox, which is similar to an MMDF mailbox. Mail
messages start with a line containing only '*** EOOH ***' and
end with a line containing only '\037\014'.