CrossBox on a server that has Postfix and Dovecot?
1. Can I install CrossBox on a server that already has PostFix and Dovecot?
2. Can we use CrossBox as a simple Webmail UI?
3. Can we have CrossBox automatically create user accounts upon successful IMAP login?
If you install CrossBox on the server which has IMAP/SMTP services already running, those will be used automatically by CrossBox and it won't change any of your existing configuration files.
3. Yes, new users are automatically created (including the database) if the IMAP login works.