Exchange 2007 Back Presure

I am trying to find information concerning the Back Presure - Version buckets. I have looked at http://technet.microsoft.com/en-us/library/bb201658.aspx but it really does not explain the Version buckets.


Event Type:      Warning
Event Source:      MSExchangeTransport
Event Category:      ResourceManager
Event ID:      15004
Date:            7/7/2009
Time:            1:46:08 PM
User:            N/A
Computer:      HUB-CAS
Description:
Resource pressure increased from Normal to Medium.

Resource utilization of the following resources exceed the normal level:
Version buckets = 171 [Medium] [Normal=80 Medium=120 High=200]

Back pressure caused the following components to be disabled:
Inbound mail submission from the Internet
Mail submission from the Pickup directory
Mail submission from the Replay directory
Mail delivery to remote domains

The following resources are in the normal state:
Queue database and disk space ("C:\Program Files\Microsoft\Exchange Server\TransportRoles\data\Queue\mail.que") = 8% [Normal] [Normal=95% Medium=97% High=99%]
Queue database logging disk space ("C:\Program Files\Microsoft\Exchange Server\TransportRoles\data\Queue\") = 8% [Normal] [Normal=95% Medium=97% High=99%]
Private bytes = 7% [Normal] [Normal=71% Medium=73% High=75%]
Physical memory load = 38% [limit is 94% before message dehydration occurs.]