Exchange 2013 hybrid mail flow breaks if old domain controller (2008) is taken offline

Archil Berikishvili 1 Reputation point

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?

Active Directory
Active Directory
A set of directory-based technologies included in Windows Server.
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Microsoft Exchange Hybrid Management
Microsoft Exchange Hybrid Management
Microsoft Exchange: Microsoft messaging and collaboration software.Hybrid Management: Organizing, handling, directing or controlling hybrid deployments.
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  1. Archil Berikishvili 1 Reputation point

    @Andy David - MVP this event doesn't show up on edge server, on mailbox servers it shows:

    dc1 CDG 1 7 7 1 0 1 1 7 1
    dc2 CDG 1 7 7 1 0 1 1 7 1
    dc3 CDG 1 7 7 1 0 1 1 7 1

    dc1 is the one we're trying to decommission

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  2. Andy David - MVP 109.9K Reputation points Microsoft MVP

    Ok, when you take the DC offline, does it affect all mail flow or just mail flow to 365? Which server are the messages queued on and what is the error in the queue and in the event logs?
    Nothing is hardcoded to that DC is it?

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