Exchange receive connectors do have a setting of MaxInboundConnectionPerSource which limits the maximum number of connections from a single IP address at any moment. So with a default value of 20, there are no more than 20 connections from a specific IP address allowed. The following warning is logged in the transport server's application log if the limit is exceeded:
Event ID: 1021
Event Source: MSExchangeTransport
Message Text: Receive connector 'Connector Name' rejected an incoming connection from IP address '10.10.10.10'. The maximum number of connections per source ('20') for this connector has been reached by this source IP address.
The sender of a mail might receive the following message:
Deferred: 421 4.3.2 The maximum number of concurrent connections has exceeded a limit, closing transmission channel" Status 4.0.0, "Transient failure"
According to RFC this is not a permanent error, so the sender's mail server should try to send the message again after some delay.
"The command was not accepted, and the requested action did not occur. However, the error condition is temporary, and the action may be requested again."
To view the configured value of MaxInboundConnectionPerSource use the Get-ReceiveConnector cmdlet, to change the value use the Set-ReceiveConnector cmdlet with the –MaxInboundConnectionPerSource parameter.
The following example increases the value to 100
Get-ReceiveConnector default* | Select Name,MaxInbound*
Get-ReceiveConnector default* | Set-ReceiveConnector -MaxInboundConnectionPerSource 100
More info on: TechNet