maximum number of connections per source (20) for connector has been reached

Issue: You are having issues with inbound mail or receive the following error in the application event log: “maximum number of connections per source (20) for connector has been reached”

Background: Your most likely to see this error on receive connectors that are receiving mail from your spam filtering solution.  This is because your spam filter likely sends large batches of messages in each SMTP conversation.  The receive connector getting mail from your spam filter is in theory receiving trusted mail from the the spam filter and shouldn’t be capping out at 20.

Resolution: Increase receive connector “max inbound connections per source”

Original value:
MaxInboundConnectionPerSource           : 20

New Value:
MaxInboundConnectionPerSource           : 1024

Command used:
Set-ReceiveConnector IronPort -MaxInboundConnectionPerSource 1024

Full event log error:
Log Name:      Application
Source:        MSExchangeTransport
Event ID:      1021
Task Category: SmtpReceive
Description:
Receive connector IronPort rejected an incoming connection from IP address 192.168.x.x. The maximum number of connections per source (20) for this connector has been reached by this source IP address.

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