Description of problem:
Please remove PrivateDevices=true and PrivateNetwork=true from systemd unit file:
RHEL 7 is based on Fedora 19 and the systemd version shipped with RHEL 7 doesn't support these systemd Features.
This is only for F21+
Version-Release number of selected component (if applicable):
I've submitted amavisd-new-2.9.1-4.el7 with PrivateDevices and PrivateNetwork removed.