Back Pressure and Exchange Server Transport error 452 4.3.1 Insufficient system resources

by Bharat Suneja

In my hotel room in Orlando, getting ready for an Exchange 2007 transport presentation tomorrow morning at TechMentor. When trying to telnet to the SMTP port of an Exchange 2007 Hub Transport server, I got the following error:

452 4.3.1 Insufficient system resources

Not a good thing the night before a presentation – Murphy’s law at work again!

The Application Event Log has Event ID 15002 from MSExchangeTransport saying “The resource pressure is constant at High. Statistics… “. The event goes on to tell you that inbound mail submission has stopped, and it’s due to disk space being low on the volume where the queue database is located.

Figure 1: Event ID 15002 logged by MSExchangeTransport

What’s changed in Exchange 2007 and later?

In Exchange 2007, Microsoft wrote a brand new SMTP stack for Exchange Server instead of relying on the SMTP Server in IIS, which historically came with its own dependencies and soft spots. An important part of the change was the move of message queues to an ESE database, not very different from Exchange Server’s mailbox databases. This table shows some of the differences relevant to this issue:

What In Exchange 2007/2010 In Exchange 2003
Dedicated transport server roles Hub Transport server and Edge Transport server Front-End Server (also a front-end for other protocols – IMAP/POP/HTTP)
The SMTP component Exchange has its own Uses SMTP component in IIS
Transport queues are stored in… An ESE database .EML files on the file system
Location of transport queues \Exchange Server\TransportRoles\data\Queue \mailroot\vsi <1>\queue
where <1> is the instance number of the SMTP virtual server
Built-in resource monitoring component Resource Manager None
(Admins wrote scripts or used monitoring tools)

If you’re curious to dig in further, check out the quite comprehensive Queues doc in Exchange 2013 documentation.

What’s Back Pressure?

If you’re familiar with Exchange mailbox databases, they consist of a database file and transaction logs. If the disk volume on which they reside runs out of disk space, the database dismounts.

In Exchange Server 2007 and later, the Transport service monitors system resources such as disk space and memory on Transport servers (the Hub Transport and the Edge Transport servers), and stops message submission if it’s running low on these resources. It continues to deliver existing messages in the queue. When resource utilization returns to normal, it resumes message submission. The feature is called Back Pressure (Back Pressure documentation for previous versions: Exchange 2010 | Exchange 2007 – these docs are accessible from the Other Versions drop-down from any of these links”).

In this case, Exchange required 4 GB of free disk space on the volume where the Queue database was located – I had about 3.95 Gigs. :)

Changes to Back Pressure settings in Exchange Server 2007 SP1

The Back Pressure settings in Exchange Server 2007 RTM stop inbound mailflow if free disk space is below 4 Gigs. This static threshold has been lowered in SP1 to a more realistic 500 MB.

Changes to Back Pressure settings in Exchange 2010 and later

In Exchange 2010, BackPressure settings are calculated using the formulas documented in Back Pressure, as shown in this table. High utilization is generally above 99% of total disk space, unless you’re using really small disks.

For volume hosting the… The formula… If the volume size is… High utilization is above… Free space you need is…
Message queue database 100 * (hard disk size – fixed constant1) / hard disk size 2000 GB (2 TB) 99%3 20 GB
Message queue database transaction logs 100 * (hard disk size – MIN(5 GB, 3 x DatabaseCheckPointDepthMax2)) / hard disk size 2000 GB (2 TB) 99%3 20 GB
  1. The fixed constant is 500 MB by default.
  2. The DatabaseCheckPointDepthMax key controls the total allowed size of all uncommitted transaction logs that exist on the hard drive. By default, it is 512 MB in Exchange 2010, 384 MB in Exchange 2013.
  3. The results of the formula are always rounded down to the nearest integer. By default, the medium level of hard drive utilization is 2 percent less than the high level. The normal level of hard drive utilization is 4 percent less than the high level.

Additionally, you may see the following events in the Application Event Log:

Event ID Type Source/Cateogry Description When
15004 Error MSExchangeTransport / Resource Manager Resource pressure increased from Previous Utilization Level to Current Utilization Level. Resource utilization level increases
15005 Info MSExchangeTransport / Resource Manager Resource pressure decreased from Previous Utilization Level to Current Utilization Level. Resource utilization level decreases
15006 Error MSExchangeTransport / Resource Manager 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. Available disk space is critically low
15007 Error MSExchangeTransport / Resource Manager 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. Available memory is critically low

The Resolution

Many configuration options for transport servers are saved in an XML file named EdgeTransport.exe.config located in \Exchange Server\Bin\. It’s the same file name and path on both transport server roles— Edge Transport and Hub Transport.

To get Exchange transport to resume message submissions, you can use any of the following methods. All of the following require you to edit the EdgeTransport.exe.config file.

  1. Disable BackPressure: Although Microsoft doesn’t recommend it, it does provide a way to Disable Back Pressure
  2. Tweak BackPressure thresholds: Modify BackPressure parameters to more accurately define what’s high utilization for your deployment or server configurations, as explained in the above docs.
  3. Move the queue database to another volume: Another resolution, and the one I used in this case, was to move the queue database to another volume with ample of free space, using the following procedure:
    1. Add the following key in the <AppSettings> section in EdgeTransport.exe.config, as documented in “How to Change the Location of the Queue Database“:

      <add key=“QueueDatabasePath” value=”D:\Queue\QueueDB” />

    2. Save the file and restart the Microsoft Exchange Transport service from the Services console or by using the Restart-Service cmdlet (Restart-Service MSExchangeTransport).

Message submission resumes.

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{ 36 comments… read them below or add one }

Scott Landry April 23, 2007 at 8:09 am
Florin June 27, 2007 at 6:01 am

Thank you for your post. It helped me see why in my test virtual environment I couldn’t send not even an e-mail (and was wonderring for some time now what I did wrong .. my VM had 10 Gig hard drive and it had only 3.9 Gig free :( )


TimH August 9, 2007 at 10:12 am

So what is the unit for PercentageDatabaseDiskSpaceUsedHighThreshold? I set it to 1024000000 (1GB?) and restarted MSExchangeTransport and still got 15002 errors with mail stuck in the outbox. I finally just turned off backpressure altogether but am wondering if I set the threshold incorrectly in the config file.


Bharat Suneja August 11, 2007 at 11:37 pm

Hi TimH,

Please refer to the BackPressure documentation (also linked in the above post). Look at Table2 that lits these parameters. The valid values are listed.

As the term PercentageDatabaseDiskSpaceUsedHighThreshold states, it’s not an absolute value (in memory units) but a percentage.


Anonymous December 25, 2007 at 1:14 am

Thank you for attempting, my problem was it that you solved.

Best regard

Alireza Soleimani


Saravanan.T,Chennai May 15, 2008 at 4:36 am

Thank for your Post.Previously i am not able send mails after reading your post i rectified problem.Thank you very much.


GuruLeeNyc July 27, 2008 at 12:01 pm

This posting was a godsend, it resolved an issue halting email between by Exch2007 & 2003 Interop Connector!



Michael January 7, 2009 at 4:53 am

Thanks! Moving the queue DB and logs fixed the issue for me as well.


justyn bridge March 21, 2009 at 6:24 am

Hi Bharat, thanks for your blog. It has been interesting reading. I have a similar issue and at this point am unsre if it is related to system resources or lack of. Here is the issue:

From Exchange 2007 server i cannot telnet to any remote domains on port 25.NSLOOKUP queries resolve DNS and MX records ok for these domains from the exchange server. From a second server on the same network i can telnet to domain mx records on port 25 without any issues. It is only from the exchange server i cannot telent out to port 25. I can ping the domain names ok from the exchange server so my DNS looks ok. There is no AV or Firewall software located on this server.
I noticed the following error in the event log however.
Your system is low on virtual memory. Windows is increasing the size of your virtual memory paging file. During this process memory requests for some applications may be denied.
Our paging file utilisation was at about 90% but have increased paging file in accordance with nest practice analyzer ( 8200mb )I have at least 50GB free disk space. Has anyone seen this before? Is it possible the SMTP stack is corrupt? I can recieve e-mail ok but have had to use IIS SMTP connector on another box to deliver e-mail. Any ideas??


Paul MacNeill April 17, 2009 at 7:18 pm

Solved the issue for me in EBS, TMG server, moved the Queue to a different volume as per the MS Article, and queues are now operational without the BackPressure Issue.


jmelashenko May 11, 2009 at 11:37 am

Thanks Bharat for your post. This came in really handy this morning!


CarecA May 17, 2009 at 10:32 am

Very useful tip. Thanks a lot.


web design India June 22, 2009 at 10:16 pm

very cool post


Anonymous August 12, 2009 at 3:32 pm

Very good post. Thanks a lot.


Anonymous September 1, 2009 at 5:20 pm

Woot! Thanks, that saved my bacon. I was able to get the fix in before any of our lawyers saw that they hadn't received emails for 1/2 an hour….


Anonymous November 13, 2009 at 5:08 pm

You are a star


paul August 30, 2010 at 10:19 pm

Thanks for the post. Fixed out issue as well. Although interestingly I got no errors in the event logs, just the SMTP error (452 4.3.1 Insufficient system resources) when I used telnet to connect directly to the SMTP interface on the exchange server and did a “mail to:” command.


Mario Di Vece November 13, 2010 at 2:40 pm

Thanks so much! Worked like a charm.


sanka June 20, 2011 at 5:09 pm

great work friend … thank you very much..


Shannon June 30, 2011 at 9:11 am

THANKS! Solved my problem…. my problem was memory usage was up over 95% (the threshold was 94%)…. Added more RAM and problem solved.



Eisenhorn July 11, 2011 at 8:19 am

It worked! Yes I echo the other’s sentiments. Thank you for writing the article.


Dave Hildebrand December 17, 2011 at 11:59 pm

My issue was due to system drive free space. It had just inder 1.9GB free. Disk cleanup free’d enough space to start processing external mail again. Internal mail was working normally.

I’ve since moved the DB and logs to another drive with plenty of space.


Sachin Mulekar December 22, 2011 at 3:23 am

Please Try this First and then try the other methods

In this case, Exchange required 4 Gigs of free disk space on the volume where the Queue database was located – I had about 3.95 Gigs. :)

Thanks & Regards
Sachin Mulekar


Lupo February 1, 2012 at 12:51 pm

I had the exact same problem, searched for hours but my Exchange had enough free space on all drives, no errors in the log – I couldn’t find the problem!
Then I realized that our firewall is “Forefront Threat Management Gateway” 2010 with “Forefront Protection 2010 for Exchange Server” installed.
So some kind of transport is running on our TMG which had very low free space on the C: drive.
Increased the free space, restarted – and now everything works fine again.
So don’t forget about TMG if you run into this error!


lhenry August 28, 2012 at 12:50 am

r you dumb ? It’s not murphy’s law .. it’s Microsoft .. ha aha ha


S. Kolic November 5, 2012 at 3:17 am

Thanks for this post. We had this issue today. I’ve moved the Queue to another disk with more disk space. Resolution 3 worked fine for me.


Eddy Princen January 10, 2013 at 12:50 am

Thanks a lot for this post.
We moved de db and everything ok.



Asokan February 4, 2013 at 12:52 am

After clearing some space in c drive the problem is solved.


Dave March 22, 2013 at 6:50 am

This worked for me. I had 7gb free on each of my two partitions, so i looked at virtual memory. It looked like very little was left, didn’t do the math, but changed the default threshold from 94 to 96 and restarted the hub transport service and poof. Mail started flowing in. I’ll have to look into a more permanent solution


Warren April 23, 2013 at 1:33 pm

I tried this solution and was getting an Event Log error stating the path didn’t exist. So I moved the TransportRoles folder to the path I had referenced in EdgeTransport.exe.config (on a volume having more space).

Big mistake. Next Event Log error was re: unable to create log file (in original path in TransportRoles folder). So I copied the folder back to original location.

Now, I get event 16023: Microsoft Exchange couldn’t start transport agents. The Microsoft Exchange Transport service will be stopped. Exception details: No valid agents.config file was found in ‘C:\Program files\Microsoft\Exchange Server\TransportRoles\Shared\agents.config’. The default configuration for transport agents can be recovered by running ‘scripts\ReinstallDefaultTransportAgents.ps1′. —> System.IO.FileNotFoundException: Unable to find the specified file.
— End of inner exception stack trace —
at Microsoft.Exchange.Data.Transport.Internal.MExRuntime.MExConfiguration.Load(String filePath)
at Microsoft.Exchange.Data.Transport.Internal.MExRuntime.MExRuntime.Initialize(String configFile, String agentGroup)
at Microsoft.Exchange.Protocols.Smtp.MExEvents.Initialize(String configFilePath)
at Microsoft.Exchange.Transport.Components.SmtpMExRuntimeLoader.Load()

Having run the suggested script and still getting the same message, I’m wondering if it is necessary to re-register the dlls referenced in that agents.config file, or if there is some other explanation. It is strange because the file did exist (after copying it back) and certainly the default configuration exists after running the script. Yet the message continues to say FileNotFoundException.


Warren April 23, 2013 at 2:27 pm

Solved it myself– the Exchange Transport service was running with NT AUTHORITY\NetworkService. When I moved the files back and forth I lost the permissions for that account. Trying to re-add them though, I couldn’t find NetworkService. I had to enter “Network Service” (with a space).

Hope this helps someone with a similar problem.


Jorge Gaitan January 7, 2015 at 7:49 am

thank you very much for this Article. saved the Day.
can I change the queue live? or should I shutdown exchange services before making the queue change?
the exchange database is going to keep growing so it makes no sense for me to keep the queue on the same drive.
but MGMT is not so sure to make the change now.


Bharat Suneja January 7, 2015 at 9:41 am

To move the queue, you’ll need to restart Transport service. You do not need to stop all services or mailbox databases. See Change the location of the queue database.

Note, in Exchange 2013 the transport queue is also a database. Exchange will create a new database in the new path.


Andreas Will December 21, 2016 at 7:01 am

hello, i got the problem today, could not receive mails any more. sending mails was no problem. on volume there have been “only” 50GB free disk space, where total space is 1,82TB. i moved some data to another volume, in the end i had about 100GB free disk space on the volume. than receiving mails was no more a problem, all works fine. but i absolutely don´t unterstand why there is, in my case, about 100GB of free disk space necessery that i can receive mails,or, in other cases, the error display above is not appearing. can somebody explain, why there is so much free disk space needed? is 50GB free disk space not enough? thanks, kind regards, andreas


Bharat Suneja December 21, 2016 at 11:57 am

Based on the formula used to calculate BackPressure settings, you would require approx 99.97 GB of free disk space for a drive of that size (1820 GB). It’s close to or little over 5 percent.

You can configure the settings.

The post has been updated to include the formulas used in Exchange 2010 and later.


Jose Gabriel Ortega Castro January 30, 2017 at 8:13 pm

This happens to me yesterday. This error happens because the lack of free space to make the succesfully delivery of the emails.
So the resolution is:
1 Check the Local space for the logs and for the edb (files on the exchange server)
To meet this you can use this powershell script:
Please do not modify exchange server edgetransport.exe.config files since it’s a bad practice, it’s just a workaround is not a solution.

2. Enable circular logs on the Exchange server Databases and then “Unmount” and “Mount” them again. you will get your emails flowing really fast.

3. I will recommend also using my script Gives you the location of those Database in the Filesystem (Path) you will need to create a backup (enabling the “VSS Full Backup” option in the VSS Settings tab) as showed here to clean up even more the logs space on the exchange DB (

Jose Gabriel Ortega C
MCSE/MCP Messaging /Server Administrator
Top rated Upwork Freelancer


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