In Microsoft Exchange Server 2019 and Exchange Server 2016, threads of MSExchangeMapiMailboxAppPool are stuck in lock contention (clr!AwareLock::Contention), which causes the affected server to stop responding with 100% CPU usage for a long time (1+ hour).
To fix this issue, install one of the following updates:
For Exchange Server 2019, install the Cumulative Update 7 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019.
For Exchange Server 2016, install the Cumulative Update 18 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
Learn about the terminology that Microsoft uses to describe software updates.