I suppose that Dovecot and Postfix are up and running, and you can receive and send mail with system user (see previous posts). It is time to configure authentication in LDAP.
Use of directory service to user authentication allows for flexible management of mail system, hosting and so on. LDAP is established standard for authentication and authorization and almost all software which requires authentication support this protocol.
Let’s begin from POP3/IMAP Dovecot server, which also deliver authentication mechanism for Postfix:
/usr/bin/sudo -i cd /etc/dovecot vi dovecot-ldap.conf