Master server vs. slave server

Question

What does the master server do vs. just a normal server?

Answer

Master server:

• hosts frontend application (CrossBox client)
• proxies IMAP/SMTP/POP3 traffic to the correct slave (CrossBox MailProxify)
• enables one webmail/IMAP/SMTP/POP3 hostname for all users (no matter on what slave they are actually hosted)
• routes logins to the correct slave
• forwards backend/frontend updates to slaves
• delivers updates to iOS/Android users

Slave server:

• hosts backend (CrossBox server) and other services for accounts on that server
• stores data (emails, files, databases) for accounts on that server
• does all work for accounts on that server