Maildir is an e-mail spool format that keeps all mails in seperate files, and thus does not need file locking to maintain message integrity. All operations are atomic.
A MailDir directory contacins thgree subdirectories: tmp, new and cur. The populair Courier IMAP/POP servers work with the MailDir format.
sudo postconf -e "home_mailbox = Maildir/"Restart postfix to make the changes take effect.
#systemctl restart postfix