It seems to be a critical issue in version 5.10.
Reference Number Related Article Issue Description 120314 - Issue: DHCP relay feature does not work with DHCP servers that drop relay agent information option from the DHCP messages. For example, Windows 2008 DHCP behaves this way.
that will be fixed in 5.10.1, scheduled for release in end of January. The issue is that firewall includes option 82 in the DHCP packets but some servers do not include it in their reply if they don't support it. Win2008 is one of these servers: http://blogs.technet.com/b/teamdhcp/archive/2012/10/01/option-82-support-in-dhcp -server-for-the-ip-source-guard-scenario.aspx