Email Subscription Service Failures

We are currently using the On-Premise (August 2018) verison of Power BI Report Server.  We routinely (daily) experience failures of numerous subscription reports during various times of the day/night.  There are times when several of the same reports fail daily but others may fail also. The failures are intermittent and don't exhibit any specific pattern. When we re-schedule the reports to run almost immediately, sometimes we receive subsequent failures and sometimes the reports running successfully.

The following error message shows up for each failure:

ERROR: Error sending email. Exception: System.AggregateException: One or more errors occurred. ---> System.Net.Mail.SmtpException: The server committed a protocol violation The server response was:

   at System.Net.Mail.SendMailAsyncResult.End(IAsyncResult result)

   at System.Net.Mail.SmtpClient.SendMailCallback(IAsyncResult result)

   --- End of inner exception stack trace ---

   at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)

   at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)

   at System.Threading.Tasks.Task.Wait(TimeSpan timeout)

   at Microsoft.ReportingServices.EmailDeliveryProvider.EmailProvider.Deliver(Notification notification)

---> (Inner Exception #0) System.Net.Mail.SmtpException: The server committed a protocol violation The server response was:

   at System.Net.Mail.SendMailAsyncResult.End(IAsyncResult result)

   at System.Net.Mail.SmtpClient.SendMailCallback(IAsyncResult result)<---

. Additional Information: SmtpException StatusCode:GeneralFailure

Unfortuantely, the error is very vague and is extremly difficult to troubleshoot - even with Verbose logging. Due to this on-going problem, we have to re-run up to about 20 reports daily. We have conducted extensive research and analysis of our report and SMTP server configurations but are still unable to resolve this annoying problem.

If anyone has experienced this issue and/or can provide some insight on how to permanently solve it, please let me know as soon as possible.