How to split mailbox into single file per message?
Just use formail. formail is a program that can process mailbox, run some actions for each message in the mailbox, separate messages and so on.
More info: http://www.manpagez.com/man/1/formail/
If you want just split a mailbox to separate files,I would suggest such solution:
$ cat $MAIL | formail -ds sh -c 'cat > msg.$FILENO'From man:
FILENO While splitting, formail assigns the message number currently being output to this variable. By presetting FILENO, you can change the initial message number being used and the width of the zero-padded output. If FILENO is unset it will default to 000. If FILENO is non-empty and does not contain a number, FILENO gen- eration is disabled.
If you don't have formail, you can also use this Perl oneliner (copied from here, and just tested on a Thunderbird Inbox I needed to split)
perl -pe 'open STDOUT, ">out".++$n if /^From /' < $IN > before_firstor, to have 0-padded numbers:
perl -pe 'open STDOUT, sprintf(">m%05d.mbx", ++$n) if /^From /' < $IN > before-first