mb2md binary package in Ubuntu Focal s390x

 mb2md (mbox to maildir) takes one or more Mbox format mailbox files
 in a directory and convert them to Maildir format mailboxes.
 .
 As the Mbox format has some drawbacks, D. J. Bernstein created the
 Maildir format when he wrote Qmail. 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 separate 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 many 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 or any MTA which can use procmail as
 MDA. POP3/IMAP servers which support Maildirs are e.g. Courier IMAP
 and Dovecot.
 .
 mb2md does not only convert Mbox mailbox files into a Maildir but can
 also convert 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.

Source package