Recently in one of the Exchange 2013 environment POP3 clients started getting the problems in downloading the emails from the server.

The strange issue was users were unable to download the emails intermittently and it was not permanent for POP3 accounts.
When this issue occurs the POP3 accounts will stall for a while and later after some time it would start collecting the emails from the server without any issues.

This really looked strange and inorder to troubleshoot further enabled the POP3  protocol logging by the below command

Set-POPSettings -Server “CAS01” -ProtocolLogEnabled $true

After a while looked into the POP3 logging and strange to see the below message

This issue is happening, because the POP3 connections are sending more requests to the server.

This  is the main reason for the application to intermittently drop the connection.

When the connection limit  per user exceeds the default allowed limit, the connection will be forcibly closed by the mail server. And then this connection reset will happen after 4 minutes after which the client can reestablish the connection , download the emails until it reaches the threshold limit of per user.

The default value for the single user is 16

Can be seen from EAC – Servers – Edit – POP3 

Also can be seen by  running Get-POPSettings | fl

So the POP3 Throttling policy allow the counter reset after 24000 milliseconds . So when the user connection limit exceeds the default value he wouldn’t be able to connect till the next counter reset happens.

Solution:

So the POP Connection limit can be increased by running the below command

Set-POPSettings -MaxConnectionsPerUser  “connectionvalue”

Its important to note that both the POP services POP& backend needs to be restarted after this change to take effect,so we can go ahead and run the below command for the restart of the services.

Get-Service *POP* | Restart-Service

Additional Info:

The POP3 throttling policy value can be seen by running the below command:

Get-ThrottlingPolicy -Identity Default* | fl POP*

POPMaxConCurrency – The PopMaxConcurrency parameter specifies how many concurrent connections a POP user can have against an Exchange server at one time. A connection is held from the moment a request is received until a response is sent in its entirety to the requestor.
POpMaxBurst-  The PopMaxBurst parameter specifies the amount of time that a user can consume an elevated amount of resources before being throttled.
POPRechargeRate – The PopRechargeRate parameter specifies the rate at which the user budget is charged back
POPCutoffBalance – The PopCutoffBalance parameter specifies the resource consumption limits for a user before that user is completely blocked from performing operations on a specific component.

There were Get-WorkloadPolicy IMAP,POP commandlets present before Exchange 2013 CU6 , but later now these commandlets have been removed post CU6 and replaced with Set-SettingsOverride but strictly this Set-SettingsOverride command should be used only under the supervision of Microsoft Support professional.

These values also can be modified based on the requirement , just in case if we have any applications which requires these values to be modified as per the requirement.

Thanks & Regards
Sathish Veerapandian