Interesting one in Exchange 2010 at a client yesterday. The queue report starting showing Mapi delivery failures to a certain store, upon investigation the error above was encountered:


432 4.3.2 STOREDRV.Deliver; recipient thread limit exceeded

Upon checking the queue, only 1 mailbox was receiving all this mail.

Back in SP1 for Exchange 2010, Microsoft introduced Store Driver Fault Isolation and this prevents a mailbox from being hit with a large volume of mail at a time. After about 15 minutes the queues dropped again, but you can make an organization change to increase this.

You need to modify the EdgeTransport.exe.config file located in the bin folder in the Exchange Installation directory.

Now add in the following 2 keys:

<add key="RecipientThreadLimit" value="2" />  
<add key="MaxMailboxDeliveryPerMdbConnections" value="3" />

Once done, save the file and then restart the Transport Service.

Hope it helps.

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