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