ISPMan is a distributed system to manage components of ISP from a central management interface.
These components run accross frontend and backend servers.
Front end servers are the machines that are directly visible to your internet users. For example your web (eg. Apache), SMTP (eg. Postfix), DNS (eg. Bind) servers while backend servers can be Mailbox servers (eg. Cyrus IMAP server), Fileservers, database servers etc.
ISPMan is designed to be scalable.
You may start with a single server to manage user's mailboxes and add more as you grow. ISPMan can manage this and allow you to create user's accounts and mailboxes on different servers. This does not affect the user at all but allows the system administrator to balance the load of mails on different machines.