Intermittent mail sending failure - net_io_connectionclosed

Question

I am using:
SQL Server 2012 (Service Pack 2 - Business Intelligence Edition)  11.0.5058
.Net Version 4.5.1 (378675)
Windows Server 2012 R2 Standard
Exchange Server 2010 R2 (Hosted on a seperate server)
I am getting the following error in the Reporting Services log intermittently for reports:
emailextension!WindowsService_15!7cc0!10/16/2014-06:05:33:: e ERROR: Error sending email. Exception: System.Net.Mail.SmtpException: Failure sending mail. ---> System.IO.IOException: Unable to read data from the transport connection: net_io_connectionclosed.
   at System.Net.Mail.SmtpReplyReaderFactory.ProcessRead(Byte[] buffer, Int32 offset, Int32 read, Boolean readLine)
   at System.Net.Mail.SmtpReplyReaderFactory.ReadLines(SmtpReplyReader caller, Boolean oneLine)
   at System.Net.Mail.SmtpReplyReaderFactory.ReadLine(SmtpReplyReader caller)
   at System.Net.Mail.CheckCommand.Send(SmtpConnection conn, String& response)
   at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception)
   at System.Net.Mail.SmtpClient.Send(MailMessage message)
   --- End of inner exception stack trace ---
   at System.Net.Mail.SmtpClient.Send(MailMessage message)
   at Microsoft.ReportingServices.EmailDeliveryProvider.EmailProvider.Deliver(Notification notification)
notification!WindowsService_15!7cc0!10/16/2014-06:05:33:: i INFO: Notification d4b91fc4-0cc5-464a-a927-af8c24e4fcc8 completed.  Success: True, Status: Failure sending mail: Failure sending mail., DeliveryExtension: Report Server Email, Report: Missed NDD
Despatch, Attempt 0
They try to send the mail via an exchange server that is hosted on a separate server, the report server email is set up as:
<Extension Name="Report Server Email" Type="Microsoft.ReportingServices.EmailDeliveryProvider.EmailProvider,ReportingServicesEmailDeliveryProvider">
                <MaxRetries>3</MaxRetries>
                <SecondsBeforeRetry>900</SecondsBeforeRetry>
                <Configuration>
                    <RSEmailDPConfiguration>
                        <SMTPServer>****.****.****</SMTPServer>
                        <SMTPServerPort>
                        </SMTPServerPort>
                        <SMTPAccountName>
                        </SMTPAccountName>
                        <SMTPConnectionTimeout>
                        </SMTPConnectionTimeout>
                        <SMTPServerPickupDirectory>
                        </SMTPServerPickupDirectory>
                        <SMTPUseSSL>
                        </SMTPUseSSL>
                        <SendUsing>2</SendUsing>
                        <SMTPAuthenticate>
                        </SMTPAuthenticate>
                        <From>****.****@****.****</From>
                        <EmbeddedRenderFormats>
                            <RenderingExtension>MHTML</RenderingExtension>
                        </EmbeddedRenderFormats>
                        <PrivilegedUserRenderFormats>
                        </PrivilegedUserRenderFormats>
                        <ExcludedRenderFormats>
                            <RenderingExtension>HTMLOWC</RenderingExtension>
                            <RenderingExtension>NULL</RenderingExtension>
                            <RenderingExtension>RGDI</RenderingExtension>
                        </ExcludedRenderFormats>
                        <SendEmailToUserAlias>True</SendEmailToUserAlias>
                        <DefaultHostName>
                        </DefaultHostName>
                        <PermittedHosts>
                        </PermittedHosts>
                    </RSEmailDPConfiguration>
                </Configuration>
            </Extension>
I have starred out some details for anonymity. I have other servers with a matching configuration file as above (although on a 2008 version of SSRS) that never have this problem.
The issue is intermittent and doesn't affect one specific report and on some occasions the retries work however quite often they don't work at all. I have had the network guys here monitor the port to see if the mail is even leaving the server and apparently
the failed attempts do not register any data in wire shark. I don't believe this to be a size issue with the reports as they aren't supposed to deviate much.
Any help would be much appreciated.
Dave T

Answer 1

Hi behavedave,
According to your description, you have data-driven subscription failure randomly. Right?
In this scenario, there's nothing wrong on your subscription configuration. Since the issue happens randomly, we can hardly to figure out reason directly. Here are two articles about subscription monitoring and troubleshooting. Please refer to links below:
Monitoring and Troubleshooting Subscriptions
Troubleshooting Subscriptions: Part II, Using the Reporting Services Trace Log File
If you have any question, please feel free to ask.
Best Regards,
Simon Hou