Hello,
This parameter MaxPerDomainOutboundConnections specifies the maximum number of connections that an Internet-facing Hub Transport server or Edge Transport server can have open to any single remote domain. The outbound connections to remote domains occur by
using Send connectors that exist on the server. The default value of the MaxPerDomainOutboundConnections parameter is 20.

This parameter PickupDirectoryMaxMessagesPerMinute specifies the rate of message processing for both the Pickup directory and Replay directory. Each directory can independently process message files at the rate that's specified by the PickupDirectoryMaxMessagesPerMinute
parameter. By default, the Pickup directory can process 100 messages per minute, and the Replay directory can process 100 messages per minute at the same time.
You can know more detail from this document:
Understanding Message Throttling

http://technet.microsoft.com/en-us/library/bb232205.aspx

In my opinion, if you don't have special requirement, you can just use it default settings.
Thanks,
EvanEvan Liu
TechNet Community Support

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Hope this can give you some help:
Effects of Throttling on Your Deployment in Exchange 2010 SP1

http://blogs.technet.com/b/exchange/archive/2010/08/27/effects-of-throttling-on-your-deployment-in-exchange-2010-sp1.aspx

smart

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We have 25K inbound + 40K outbound = Total 65K msg/hour
MaxConnectionRatePerMinute is 1200/min or 72k/hour - Which is enough for 25K inbound?
MaxOutboundConnections is 1000 at the same time, which should be sufficient for 40K/hour outbound, unless there is large burst of outbound connection in short interval.
MaxPerDomainOutboundConnections is 20. What is the recommendation?
PickupDirectoryMaxMessagesPerMinuteis 100/min, or 8/5sec. What is the recommendation?Kelvin Teang

There is an amazing pack of free network admin tools. click here to download it

Hello,
This parameter MaxPerDomainOutboundConnections specifies the maximum number of connections that an Internet-facing Hub Transport server or Edge Transport server can have open to any single remote domain. The outbound connections to remote domains occur by
using Send connectors that exist on the server. The default value of the MaxPerDomainOutboundConnections parameter is 20.

This parameter PickupDirectoryMaxMessagesPerMinute specifies the rate of message processing for both the Pickup directory and Replay directory. Each directory can independently process message files at the rate that's specified by the PickupDirectoryMaxMessagesPerMinute
parameter. By default, the Pickup directory can process 100 messages per minute, and the Replay directory can process 100 messages per minute at the same time.
You can know more detail from this document:
Understanding Message Throttling

http://technet.microsoft.com/en-us/library/bb232205.aspx

In my opinion, if you don't have special requirement, you can just use it default settings.
Thanks,
EvanEvan Liu
TechNet Community Support

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