A lots of mails are stuck in queue

I see a lots of mail are stuck in queue. When check the error, gets that " 432 4.3.2 STOREDRV .Deliver; Recipient threat limit exceeded"

However, mails are releasing slowly, what is this error, can someone please explain?
Arif KhanSystem AdministratorAsked:
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Jose Gabriel Ortega CastroCEO Faru Bonon IT - EE Solution ExpertCommented:
All mailboxes have a quota limit.
That particular mailbox or more than one reached the mailbox quota:
you can get the quotas using Get-mailbox
help: https://technet.microsoft.com/en-us/library/bb123685(v=exchg.160).aspx

And you can Set the quota limits in here:

Or in the Database properties in exchange server.

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Ganesh GuruduSenior ConsultantCommented:
Hi Arif,

There is a not yet publicly documented key you can add to EdgeTransport.exe.config to up the number of threads used to deliver mail and to up the number of database connections.
    <add key="RecipientThreadLimit" value="2" />
    <add key="MaxMailboxDeliveryPerMdbConnections" value="3" />
The defaults for these keys when undefined are 1 and 2 receptively.  

or refer. similar issue already resolved.
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