The mailbox is in the
Inbox file, using the mboxrd format. The format is very simple; all messages are simply concatenated together, with
From_ headers added and with existing such lines in the body escaped.
Inbox file can be given to any "mbox to eml" or "mbox to Maildir" conversion tool, or even to a simple script that splits upon
/^From / lines.
Randomly-accessing a mbox mailbox can be very slow when it grows large, so
Inbox.msf is a "summary" of the mailbox in the Mork format (which is not much faster).
.msf files are only useful to Thunderbird and can be discarded otherwise.