As part of a bigger Microsoft Exchange migration, one of my customers moved the in- and outbound mailflow to a newly installed mail relay cluster. We modified MX records to move the mailflow to the new mail relay, because the customer also switched the ISP. While changing the MX records for ~40 domains, and therefore more and more mails received through the new mail relay cluster, we noticed events from MSExchangeTransport (event id 1021):
Receive connector Default Frontend EXCHANGE rejected an incoming connection
from IP address 192.168.xxx.xxx. The maximum number of connections per source
(20) for this connector has been reached by this source IP address.
192.168.xxx.xxx is the mail relay cluster, which is used for the in- and outbound mailflow.
This event indicates that the remote server has reached the maximum number of simultaneous incoming connections to the receive connector. This value is specified by the MaxInboundConnectionPerSource parameter, and the default value is 20. You can easily increase the value using the Set-ReceiveConnector cmdlet.
Set-ReceiveConnector - identity "Default Frontend EXCHANGE" -MaxInboundconnectionPersource 100
Microsoft has decreased this value over time. It was 100 in Exchange 2007, but 20 since Exchange 2010.