Exchange 2016 Back Pressure

This morning I wasnt recieving any external emails so I check my exchange 2016 box and notice this error

Event 15006, MSExchangeTransport

Microsoft Exchange Transport is rejecting message submissions because the available disk space has dropped below the configured threshold.The following resources are under pressure:
Used disk space ("D:\Microsoft\Exchange Server\V15\TransportRoles\data\Queue")
Used disk space ("D:\Microsoft\Exchange Server\V15\TransportRoles\data")
Overall Resources

The following components are disabled due to back pressure:
Mail resubmission from the Message Resubmission component.
Mail submission from Pickup directory
Mail submission from Replay directory
Mail resubmission from the Shadow Redundancy Component
Inbound mail submission from the Internet

The following resources are in normal state:
Private bytes
Queue length[Submission]
System memory
Version buckets[D:\Microsoft\Exchange Server\V15\TransportRoles\data\Queue\mail.que]
Queue database and disk space ("D:\Microsoft\Exchange Server\V15\TransportRoles\data\Queue\mail.que")

My D: drive is 602GB and I have 96GB free so I'm sure why this was back pressure was triggered or I'm just not understanding the formula.

I plan on adding addition disk space to this drive but I would like to understand what the threshold is so I dont run into this problem again.  For the moment I have it disable through the EdgeTransport.exe.config.  

Can someone tell how I can determine what the threshold for disk space?
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Todd NelsonSystems EngineerCommented:
What are the resources available on the Exchange server ... how many CPU/cores, memory, disk space available?

How is your paging file configured ... amount of RAM + 10MB?

Take a look at these references if you haven't already...
homesgAuthor Commented:
I have 4 cores, cpu is at 10%
16GB memory, setting at 79%

Disk space for my exchange drive 602GB 96GB free.

pagefile 16GB

The error is pointing to disk space and I'm trying to determine how it is calculated since I have 96GB free.
AkhaterSolutions ArchitectCommented:
The formula is

100 * (<hard drive size in MB> - 500 MB) / <hard drive size in MB>

In your case

100 * ( 602 x 1024 - 500) / (602 x 1024) = 99% which is roughly 7gb of free space you should be fine with 96 gb no matter what, are you sure it is not on another drive?

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homesgAuthor Commented:
Thats what I thought, my C: is 100GB with 28GB free.  It only has the OS installed.  

The error points to D:  

I have CU1 installed, I know CU3 has been released.  I wonder if its a bug.
AkhaterSolutions ArchitectCommented:
Really curious behavior.... Any chance you had something filling out the disk and now it's not anymore or are you still seeing this behavior?
homesgAuthor Commented:
I had to disable it through the EdgeTrasport.exe.config and restart the transporter service in order for mail to work.  its still disabled.
AkhaterSolutions ArchitectCommented:
Re reading your OP you said it is only rejecting external emails so it is in the medium range and not high

But even though for 602 gb 96% that's 24 gb free space
AkhaterSolutions ArchitectCommented:
Can you run

[xml]$bp=Get-ExchangeDiagnosticInfo -Process EdgeTransport -Component ResourceThrottling; $bp.Diagnostics.Components.ResourceThrottling.ResourceTracker.ResourceMeter

What's the output

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homesgAuthor Commented:
Resource            : PrivateBytes
CurrentResourceUse  : Low
PreviousResourceUse : Low
PressureTransitions : [PressureTransitions: MediumToHigh=75 HighToMedium=73 LowToMedium=72 MediumToLow=71]
Pressure            : 4

Resource            : QueueLength[SubmissionQueue]
CurrentResourceUse  : Low
PreviousResourceUse : Low
PressureTransitions : [PressureTransitions: MediumToHigh=15000 HighToMedium=10000 LowToMedium=9999 MediumToLow=2000]
Pressure            : 0

Resource            : SystemMemory
CurrentResourceUse  : Low
PreviousResourceUse : Low
PressureTransitions : [PressureTransitions: MediumToHigh=94 HighToMedium=89 LowToMedium=88 MediumToLow=84]
Pressure            : 79

Resource            : UsedVersionBuckets[D:\Microsoft\Exchange Server\V15\TransportRoles\data\Queue\mail.que]
CurrentResourceUse  : Low
PreviousResourceUse : Low
PressureTransitions : [PressureTransitions: MediumToHigh=1500 HighToMedium=1000 LowToMedium=999 MediumToLow=800]
Pressure            : 2

Resource            : DatabaseUsedSpace[D:\Microsoft\Exchange Server\V15\TransportRoles\data\Queue]
CurrentResourceUse  : Low
PreviousResourceUse : Low
PressureTransitions : [PressureTransitions: MediumToHigh=99 HighToMedium=97 LowToMedium=96 MediumToLow=94]
Pressure            : 86

Resource            : UsedDiskSpace[D:\Microsoft\Exchange Server\V15\TransportRoles\data\Queue]
CurrentResourceUse  : Low
PreviousResourceUse : Low
PressureTransitions : [PressureTransitions: MediumToHigh=99 HighToMedium=90 LowToMedium=89 MediumToLow=80]
Pressure            : 86

Resource            : UsedDiskSpace[D:\Microsoft\Exchange Server\V15\TransportRoles\data]
CurrentResourceUse  : Low
PreviousResourceUse : Low
PressureTransitions : [PressureTransitions: MediumToHigh=99 HighToMedium=90 LowToMedium=89 MediumToLow=80]
Pressure            : 86
AkhaterSolutions ArchitectCommented:
So now you are at low and no backpressure however you are at 86 % and backpressure will move to medium at 89% (following your output) that's 3 % breathing space which might be the trigger / issue
AkhaterSolutions ArchitectCommented:
And, as you can see,  when it moves to medium you need to go back under 80% to revert to low. I guess you had a temporary peak in the disk utilization above 86%
homesgAuthor Commented:
This makes sense, thank you.
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