SMTP alerts are generated because of Exchange 2007 maximum connection has been reached error
Description
Resolution
If you are using Exchange 2007 as your mail server, you may receive SMTP alerts from the Email Security due to "The maximum number of connections per source (20) for this connector has been reached by this source IP address" error. You can see these error in the event logs under application on the Exchange server.
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Event ID: 1021
Source : MSExchangeTransport
Type: Warning
Receive connector Internal SMTP rejected an incoming connection from IP address 192.168.1.2. The maximum number of connections per source (20) for this connector has been reached by this source IP address.
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If your inbound path on the Email Security is set to Proxy, the alerting system will basically check for SMTP connections randomly to the downstream mailserver.
If you are randomly or constantly getting alerts, "The SonicWall Email Security destination server is not responding to an SMTP test" and are using Exchange 2007 as your mail server, you can change the MaxInboundConnectionPerSource parameter on the Receive connector respectively from 1-10000. To disable the inbound connection per source limit on a Receive connector, enter a value of unlimited.
Source from Microsoft: http://technet.microsoft.com/en-us/library/bb218300.aspx