In Microsoft Exchange Server 2019 or Exchange Server 2016, you create a mailbox that's used for moderation to automatically approve or reject emails. When there're large volumes of emails, and users have to approve or reject more than one mail per second, the whole process breaks. Therefore, emails will be blocked in the Outbox folder of the arbitration mailboxes.
This issue occurs because the message throttling for sender mailboxes is enabled incorrectly in Exchange Server.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
To fix this issue, install one of the following updates:For Exchange Server 2019, install the Cumulative Update 1 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019.