Hi Arkom,
To me it seems to be the expected behavior.
If the primary domain controller (or the domain controller Exchange is currently using) become unavailable,
Exchange would lose connection with AD and it would keep trying to connect to this domain controller for about 15 minutes (you would see Event 2070 generated in the Event Viewer).
It would not only affect mail flow but also client access.
If it fails to connect during this period, Exchange would switch to connect to other available DCs found in Event 2080.
Thus if possible, it is recommended to restart the domain controller during off work time.
Another method is to specify another domain controller for Exchange to connect to, but it may also require a restart of the Exchange server to take effect.
Here is a link discussing the same issue for your reference: Exchange server stops working when Domain Controller is restart.
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