This is not only apply to Network Edition.
If you can't save in the Admin UI, maybe you didn't replace your zimbra Admin UI files. (number 20).
See this bug:
Bug 22108 - Cannot add properties to zimbraDomain object in administration console
If mail is going in/out just not getting the disclaimer signature, check for /opt/zimbra/postfix/conf/disclaimers/yourdomain.com and yourdomain.com.html