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