1021MSExchangeTransportSmtpReceiveReceive connector Allow SMTP rejected an incoming connection from IP address 1.2.3.4. The maximum number of connections per source (20) for this connector has been reached by this source IP address.
On Exchange 2010 the limit can be increased by running the following command from the Exchange shell:
Set-ReceiveConnector "MXSERVER\Allow SMTP" -MaxInboundConnectionPerSource 100
where MXSERVER is the name of the exchange server, "Allow SMTP" is the name receive connector, and 100 is the new limit.