Mail flow stops

Exchange 2013 running on virtual server Hyper V
Hyper V server windows Server 2012 R2
Exchange 2013 setup in hybrid mode with Office 365  
Mail stops flowing for no reason, I opened a ticket with Office 365 Support, and after several checks and adjustments no solution.
Last suggestion from Office 365 Support team was to check Time sync Issues/configuration on Hyper V Server.

Can this cause mail to stop flowing?
No NDR's
Solution to get mail flowing again has been to shut down Exchange down
Reboot Hyper V server and restart Exchange
Average uptime for exchange is 24 to 36 hours.

Thanks
Bert McFerrin
Bert McFerrinIT ManagerAsked:
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Scott CSenior EngineerCommented:
Yes.  Time sync can cause issues.  You want all of your server within 5 minutes of each other at the minimum.

Where is does the mail stop flowing?  Can you see it in queues?  If so, which ones?

On that server stop and restart Transport Service to see if that gets mail flowing again.

When the mail is not flowing what do the Application logs say?
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Will SzymkowskiSenior Solution ArchitectCommented:
You also need to make sure that if you are running Hyper-V that your Guest Machines are not getting their Time Synced from the Host Hyper-V server.

Will.
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Scott CSenior EngineerCommented:
^ Yes ^
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Bert McFerrinIT ManagerAuthor Commented:
When you say Guest machines do you mean the virtual servers running on the Hyper-V Server or and other servers on network
No info in  Applications Logs.
Stop and restart of transport service will not restore flow
Restart of Hyper-V server and restarting Exchange restores Mail flow
The que will have some pending status conditions but Office 365 Support could not see any meaningful info from them
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Mark DamenERP System ManagerCommented:
Check for disk space issues.  Exchange 2007 onwards really doesn't like low disk space, read about "back pressure".

Restarting exchange VM is likely reducing the size of the page file temporarily therefore allowing mail flow to resume.
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Scott CSenior EngineerCommented:
Mark is correct.

Here are my notes on Backpressure.

Back Pressure Logging Information

The following list describes the event log entries that are generated by specific back pressure events in Exchange 2010:
      } Event log entry for an increase in any resource utilization level
Event Type: Error
Event Source: MSExchangeTransport
Event Category: Resource Manager
Event ID: 15004
Description: Resource pressure increased from Previous Utilization Level to Current Utilization Level.
      } Event log entry for a decrease in any resource utilization level
Event Type: Information
Event Source: MSExchangeTransport
Event Category: Resource Manager
Event ID: 15005
Description: Resource pressure decreased from Previous Utilization Level to Current Utilization Level.
      } Event log entry for critically low available disk space
Event Type: Error
Event Source: MSExchangeTransport
Event Category: Resource Manager
Event ID: 15006
Description: The Microsoft Exchange Transport service is rejecting messages because available disk space is below the configured threshold. Administrative action may be required to free disk space for the service to continue operations.
      } Event log entry for critically low available memory
Event Type: Error
Event Source: MSExchangeTransport
Event Category: Resource Manager
Event ID: 15007
Description: The Microsoft Exchange Transport service is rejecting message submissions because the service continues to consume more memory than the configured threshold. This may require that this service be restarted to continue normal operation.

From <https://technet.microsoft.com/en-us/library/bb201658(v=exchg.141).aspx>
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Scott CSenior EngineerCommented:
Yes, Guest machines are running under your Host machines.

The mail has to be queuing up somewhere.  We need to find where and see if there is a problem where the mail is being queued up on.
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Senior IT System EngineerIT ProfessionalCommented:
Is there any Smarthost or mailgateway for filtering or relay somehow causing this issue ?
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Miguel Angel Perez MuñozCommented:
Dear Bert,

I´m bit curious, really need to restart HyperV server? I think if is Exchange related issue restarting Exchange but not HyperV server must resolve problem.

Ensure your Exchange server has not dynamic memory/oversubscription, memory ballooning, or whatever like this configured on virtual machine.
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mdvCommented:
In addition to the comments above you can try using https://testconnectivity.microsoft.com/ as well as mxtoolbox.com to see if these tools can help you locate and analyze the problem.
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Bert McFerrinIT ManagerAuthor Commented:
Thanks a lot for all the input I am going thru all suggestions.
B
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Bert McFerrinIT ManagerAuthor Commented:
Issue was resolved by correcting Time sync setting on Exchange server to point to correct domain controller.
Thanks to all for their suggestions
Bert McFerrin
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