On an Exchange 2010 server, the following error may occur in your Eventlog:
Source: MSExchangeTransport
Event ID: 1021
Task Category: SmtpReceive
Level: Warning
Computer: EXCH1
Description:
Receive connector <receive connector name> rejected an incoming connection from IP address x.x.x.x. The maximum number of connections per source (20) for this connector has been reached by this source IP address
This typically means there is a server or a process that tries to create more than 20 concurrent connections to the Receive Connector.… Read the rest