Windows Small Business Server (SBS) contains Exchange by default. If you use Exchange on your Windows SBS and have multiple DCs, you may run into the following issue.
A secondary DC shuts down, the primary DC/Exchange server is still up and running with no problems, however mail-flow in Exchange stops with no errors. The issue will seem to be resolved once the secondary DC is back up.
You have to adjust the DC that Exchange is trying to use.
- Open the Exchange Management Console.
- Right click on Microsoft Exchange On-Premises and select Properties.
- With the Properties window open, select Specify a server to connect to. Go to Browse and select the SBS Server that hosts Exchange and is also a DC.
- Shutdown secondary DCs and confirm mail now flows in both directions, internally and to outside domains.
What is happening is that Exchange is looking for a different DC rather than the local SBS Server. Exchange is looking at the secondary DC, it powers down and it no longer has access to AD and refuses to look back at the localhost server.