Log Name: Application
Date: 04/20/2012 16:15:52
Event ID: 1021
Task Category: SmtpReceive
Receive connector "Unauthenticated SMTP" rejected an incoming connection from IP address xxx.xxx.xxx.xxx. The maximum number of connections per source (20) for this connector has been reached by this source IP address.
This means that you have a large number of connections coming from the same IP address. To prevent connections from being rejected, if this volume of connections is normal, you have to change the maximum number of connections per source.
To check the current limit (20 by default), run the following cmdlet and look for MaxInboundConnectionPerSource:
Get-ReceiveConnector <server_name>\<name> | Select *MaxInbound*
To increase the limit, you have to use the Exchange Management Shell:
Set-ReceiveConnector <server_name>\<name> -MaxInboundConnectionPerSource 500 -MaxInboundConnectionPercentagePerSource 100
If you have the same receive connector in multiple servers, you can run the following cmdlet instead to update all at the same time and in one go:
Get-ReceiveConnector *\<name> | Set-ReceiveConnector -MaxInboundConnectionPerSource 500 -MaxInboundConnectionPercentagePerSource 100
Hope this helps!