Hi Matthias,
Unfortunately the ESA is not configured to support Icap capabilities to distribute traffic to other security appliances. All communication is currently done via smtp communication currently with the exception of basically pushing off log files via syslog or utilizing our enhanced Data Loss Prevention appliance (Enterprise Manager - EM).
There was an internal feature request filed for the same, however I do not see much progress on that since there were not many requests.
Thanks!
Libin Varghese