Unbound forward-zones of subdomains overlap

Question

I have a subdomain (imap) that is accessible from an external network and I have some subdomains (file-drive) that are only accessible form the internal network. I do not have access to the dns-server of the university. I tried to use different dns-server for some subdomains but that doesn't worked.

I want for imap.example.com. the dns-server 8.8.8.8 and only for the rest of the domain example.com the dns-server 192.168.1.15.

I tried the following in the unbound.conf:

server:
  use-syslog: yes
  username: "unbound"
  directory: "/etc/unbound"
  trust-anchor-file: trusted-key.key
  root-hints: "/etc/unbound/root.hints"
forward-zone:
  name: "imap.example.com."
  forward-addr: 8.8.8.8
forward-zone:
  name: "example.com."
  forward-addr: 192.168.1.15
forward-zone:
  name: "."
  forward-addr: 8.8.8.8 
remote-control:
    # Enable remote control with unbound-control(8) here.
    # set up the keys and certificates with unbound-control-setup.
    control-enable: yes

    # what interfaces are listened to for remote control.
    # give 0.0.0.0 and ::0 to listen to all interfaces.
    control-interface: 127.0.0.1

    # port number for remote control operations.
    control-port: 8953

    # unbound server key file.
    server-key-file: "/etc/unbound/unbound_server.key"

    # unbound server certificate file.
    server-cert-file: "/etc/unbound/unbound_server.pem"

    # unbound-control key file.
    control-key-file: "/etc/unbound/unbound_control.key"

    # unbound-control certificate file.
    control-cert-file: "/etc/unbound/unbound_control.pem"