We see the following error is logged on the Exchange server
Log Name: Application
Date: 25-10-2013 9:15:11
Event ID: 1021
Task Category: SmtpReceive
Receive connector Relay for printers and applications rejected an incoming connection from IP address <<your IP>>. The maximum number of connections per source (20) for this connector has been reached by this source IP address.
This is a configurable limit on the Transport Service in Exchange which is set by default to 20. You can view this setting by using the following command in Exchange Management Shell:
Get-TransportServer | FT MaxOutboundConnections,MaxPerDomainOutboundConnections
To raise the limit of 20 concurrent connections to (for example) 50 concurrent connections you can use the Set-TransportServer command in EMS:
Get-TransportServer | Set-TransportServer -MaxPerDomainOutboundConnections 50
This way the web application is able to send out (large amounts of) SMTP messages again.