This mail server allows you to send email only if you authenticate with a valid username and password, which are stored on a LDAP server. If someone sends email to one of the mail domains supported by the server, the recipient address will be checked on the LDAP directory whether it is valid, before the mail is accepted. The received emails are not stored locally, but are forwarded instead to an external email address that belongs to the user (and is recorded in the LDAP directory).
This setup could be useful for organizations or companies. It allows them to have branded email addresses and to use their own mail server for sending emails. However it spares their users from having multiple email accounts, allowing them to receive all the emails in a single mailbox, on their preferred mail provider (for example gmail.com). At the same time, it relieves the postmaster of the organization from having to manage mailboxes and backups, scanning emails for viruses and spam, etc. These are not easy tasks and they can be done much more professionally from the email providers.