Symptoms
Microsoft Exchange Server 2019 and 2016 queues that are bound to a single domain (for example, *.mail.onmicrosoft.com) will back up if a spike occurs in the mail flow or as customers migrate to Microsoft 365. In this situation, the SmtpConnectorQueueMessageCountThresholdForConcurrentConnections setting defaults to a value of 20. Per current guidance, this setting should be set to 5.
Resolution
To fix this issue, install one of the following updates:
Cumulative Update 11 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019
Cumulative Update 22 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016
References
Learn about the terminology that Microsoft uses to describe software updates.