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.

References

Learn about the terminology that Microsoft uses to describe software updates.

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