Recently, while troubleshooting an Exchange environment, I ran across event ID 2142 from the MSExchangeADTopology source. This error can be found in the application logs and indicates that the topology service could not find the minimum required domain controllers needed for Exchange. For the environment I was troubleshooting this was particularly odd as this site containing Exchange had three functional domain controllers. The environment was also a single AD site. The error in full:
Process Microsoft.Exchange.Directory.TopologyService.exe (PID=2700) Forest domain.com. Topology discovery failed, error details: No Minimal Required Number of Suitable Directory Servers Found in Forest domain.com Site Default-First-Site-Name and connected Sites.