The exchange receive connectors do have settings for Maximum Inbound connection which is set to 20 by default. If a server allowed to send emails reach this limit, you would see a log on the Exchange server.
Event 1021, MSExchangeTransport
Receive connector ‘name of connector’ rejected an incoming connection from IP address ‘server ip’. The maximum number of connections per source (20) for this connector has been reached by this source IP address.
To verify the status of the receive connector, run the following cmdlet:
Get-ReceiveConnector ‘name of connector’ | select name, maxinbound*
To adjust the limit of the receive connector, run the following cmdlet:
Set-ReceiveConnector ‘name of connector’ -maxinboundconnectionpersource <value>