What you have done is allow ALL email traffic on the 10.0.0.0/24 network to be treated as LOCAL.
Do you have a firewall between the 10.* network and the world?
My guess would be that your zimbra server is seeing all addressed that come through the Firewall as 10.* network addresses "The Firewall's address".
Put exact address for machines that you want to relay for in MyNetworks.
as zimbra user
repeat the 10.0.0.nn/32 for each server you need to relay for.
zmprov modifyserver YourZimbraServer zimbraMtaMyNetworks '127.0.0.0/8 10.0.0.nn/32