The Exchange Team wrote a post named Store Driver Fault Isolation Improvements in Exchange 2010 SP1 a while back regarding a new feature introduced in Exchange 2010 SP1 to throttle the volume of messages delivered to a single recipient.
However, this featured caused some issues for organizations with a large volume of emails to Public Folder and, more commonly, a Journal mailbox. In these cases, administrators will see mail queues with the following error:
432 4.3.2 STOREDRV.Deliver; recipient thread limit exceeded
The solution mentioned is to create the following two keys:
<add key="RecipientThreadLimit" value="2" />
<add key="MaxMailboxDeliveryPerMdbConnections" value="3" />
And add them to the EdgeTransport.exe.config file (located at ...\Program Files\Microsoft\Exchange server\V15\Bin).
However, for organizations using Exchange 2013 or 2016, these two keys need to be added to the MSExchangedelivery.exe.config file located in the same folder!
After adding the keys, restart both the Microsoft Exchange Transport (MSExchangeTransport) and Microsoft Exchange Mailbox Transport Delivery (MSExchangeDelivery) services.