Understanding back pressure in Exchange Server 2016

What is the back pressure?

Back pressure is a system resource monitoring feature of the Microsoft Exchange Transport service that exists on Mailbox servers and Edge Transport servers.

What does back pressure do?

Back pressure detects when vital system resources, such as hard drive space and memory, are over-utilized, and takes action to prevent the server from becoming completely overwhelmed and unavailable.

The following three levels of resource utilization are defined:

Low or Normal: The resource isn't overused, the server accepts new connections and messages.

Medium: The resource is slightly overused, Back pressure is applied to the server in a limited manner.

Mail from senders in the organization's authoritative domains can flow. However, depending on the specific resource under pressure, the server uses tarpitting to delay server response or rejects incoming MAIL FROM commands from other sources.

High: The resource is severely overused, full back pressure is applied.

All message flow stops, and the server rejects all new incoming MAIL FROM commands.

Monitored resources

The following system resources are monitored by back pressure:

DatabaseUsedSpace[%ExchangeInstallPath%TransportRoles\data\Queue]   Hard drive utilization for the drive that holds the message queue database.

PrivateBytes   The memory that's used by the EdgeTransport.exe process.

QueueLength[SubmissionQueue]   The number of messages in the Submission queue.

SystemMemory   The memory that's used by all other processes.

UsedDiskSpace[%ExchangeInstallPath%TransportRoles\data\Queue]   Hard drive utilization for the drive that holds the message queue database transaction logs.

UsedDiskSpace[%ExchangeInstallPath%TransportRoles\data]   Hard drive utilization for the drive that's used for content conversion.

UsedVersionBuckets[%ExchangeInstallPath%TransportRoles\data\Queue\mail.que]   The number of uncommitted message queue database transactions that exist in memory.

Source: https://technet.microsoft.com/EN-US/library/bb201658(v=exchg.160).aspx