Exchange 2007 - problem of messages getting stuck in Drafts

Problem Description

You have an Exchange 2007 installation and everything seems to be working fine. But when you sign in to Outlook Web Access (OWA) or Outlook 2007 and try to send a message, it doesn't get sent, but remains in the Drafts folder. Or you sign in using Outlook 2003 and the message remains in the Outbox folder.

If you look in the Application Event log on the Exchange server, you'll find similar warnings:

Event Type: Warning
Event Source: MSExchangeTransport
Event Category: ResourceManager
Event ID: 15002
Date: 11/25/2006
Time: 11:38:17 AM
User: N/A
Computer: HNLEX03
Description: The resource pressure is constant at High. Statistics:

Queue database and disk space ("C:\Program Files\Microsoft\Exchange Server\TransportRoles\data\Queue\mail.que") = 75% [High] [Normal=70% MediumHigh=72% High=74%]

Queue database logging disk space ("C:\Program Files\Microsoft\Exchange Server\TransportRoles\data\Queue\") = 76% [Normal] [Normal=92% MediumHigh=94% High=96%]

Version buckets = 1 [Normal] [Normal=40 MediumHigh=60 High=100]

Private bytes = 14% [Normal] [Normal=71% MediumHigh=73% High=75%]

Physical memory load = 52% [limit is 94% to start dehydrating messages.]

Inbound mail submission from other Hub Transport servers, the Internet, the Pickup directory, the Replay directory, and the Mailbox server, if it is on a Hub Transport server, has stopped. Loading of e-mail from the queuing database, if available, continues.

Event Type: Warning
Event Source: MSExchangeMailSubmission
Event Category: MSExchangeMailSubmission 
Event ID: 1009
Date: 11/25/2006
Time: 11:39:10 AM
User: N/A
Computer: HNLEX03
Description: The Microsoft Exchange Mail Submission service is currently unable to contact any Hub Transport servers in the local Active Directory site. The servers may be too busy to accept new connections at this time.

Back Pressure

The cause of this problem is a new feature in Exchange Server 2007 called Back Pressure, which is described in detail in the article Understanding Back Pressure.

It's certainly an interesting security feature for production use. It monitors the available resources on the server, such as free disk space and memory, and if the resource utilization exceeds a certain set value, it stops accepting new connections and messages. So for example, when trying a DOS attack by sending a large number of messages, when the queue is full, it will stop accepting further messages and continue only after the queue is cleared.

The default setting is that there must be at least 4GB of free disk space on the disk where the message queue is located. So if, for example, in a test virtual environment, we install Exchange 2007 on a server with less free space, no messages will be sent.

Possible Solutions

We have about three possible solutions.

Disabling Back Pressure

For production use, I wouldn't recommend this, but for testing, it's possible to disable Back Pressure. This is done by setting the configuration file EdgeTransport.exe.config, which is typically located at C:\Program Files\Microsoft\Exchange Server\Bin\EdgeTransport.exe.config.

We find the parameter EnableResourceMonitoring and change its value from true to false.

Then we restart the Microsoft Exchange Transport service.

Changing Back Pressure Parameters

Another option, which Microsoft does not recommend, is to change the parameter values for Back Pressure. Again in the EdgeTransport.exe.config file, we can set the parameters to a value of 3 to 100.

PercentageDatabaseDiskSpaceUsedHighThreshold
PercentageDatabaseDiskSpaceUsedMediumThreshold
PercentageDatabaseDiskSpaceUsedNormalThreshold

Relocating the Message Queue

We can also place the message queue on a disk with more free space. The description is in the article How to Change the Location of the Queue Database.