We have exchange 2013 cu 23 with hybrid configuration, on-premises we have couple of mailbox servers and an edge server to route emails to office 365, we also use SMG to route emails to the internet. in AD we have 3 domain controllers one of them is 2008 which we want to decommission, but mail flow from on-premises to exchange online stops if that DC is taken offline. where can we start looking at the issue? any suggestions?