mb2md.pl is a Perl script that takes one or more Mbox format mailbox
files in a directory and convert them to Maildir format
mailboxes. With the Mbox format all mail of a specific folder is
stored as one large text file. The Maildir format stores each mail as
a seperate file. It is a faster and more efficient way to store
mail. It works particularly well over NFS, which has a long history of
locking-related woes.

The Mbox format is used by most of the POP3/IMAP servers, most mail
servers (MTAs) and mail readers (MUAs). The Maildir format is used by
Qmail, Courier-MTA and can be also used as a alternative mail storage
format by Postfix and Exim. A good POP3/IMAP server for Maildirs is
Courier IMAP.

Mb2md.pl does not only convert mailbox files into a Maildir but also
the /var/spool/mail/$USER mailspool file. It is smart enough to not
transfer a dummy message such as the UW IMAPD puts at the start of
Mbox mailboxes - and you could add your own search terms into the
script to make it ignore other forms of dummy first message.