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Error 452 4.3.1

Error 452 4.3.1: Insufficient System Resources - Fix Guide 2025

Complete troubleshooting guide for Exchange Server Error 452 4.3.1 back pressure and insufficient system resources. Learn how to diagnose resource exhaustion, fix memory and disk issues, and restore mail flow in 15-30 minutes.

Medha Cloud
Medha Cloud Exchange Server Team
Exchange Database Recovery Team••14 min read

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Error 452 4.3.1 "Insufficient system resources" means Exchange Server is rejecting incoming mail because critical resources (memory, disk, or queues) are exhausted. This is Exchange's back pressure mechanism protecting itself from crashing. This guide shows you how to identify the constrained resource and restore mail acceptance.

Our Exchange Mail Flow Recovery Team resolves back pressure emergencies regularly. This guide provides the same rapid response approach we use to restore mail flow.

Error Overview: What 452 4.3.1 Means

Error 452 4.3.1 is an SMTP response code returned by Exchange when back pressure is active. The "452" indicates a temporary failure, and "4.3.1" specifically means the server lacks system resources to process the request.

Typical SMTP Session with 452 Error
220 mail.company.com Microsoft ESMTP MAIL Service ready
EHLO sender-server.external.com
250-mail.company.com Hello [192.168.1.100]
MAIL FROM:<user@external.com>
250 2.1.0 Sender OK
RCPT TO:<recipient@company.com>
452 4.3.1 Insufficient system resources

Back pressure resource monitoring:

Resource Pressure Levels

NormalResources healthy
Mail accepted normally
MediumResources constrained
New connections throttled
HighResources low
Mail acceptance slowed
CriticalResources exhausted
452 4.3.1 returned

Symptoms & Business Impact

What External Senders Experience:

  • Delayed delivery (their server retries automatically)
  • Bounce messages after extended retry period
  • SMTP logs showing 452 4.3.1 responses

What Your Users Experience:

  • Delayed incoming external email
  • Internal mail may still work (depends on which server is affected)
  • New mail appears to "not arrive"

What Admins See:

  • Event IDs 15004, 15005, 15006, 15007 in Application log
  • Back pressure status shows Critical in diagnostics
  • High memory usage by EdgeTransport.exe
  • Low disk space on queue database drive
  • Large submission queue depth

Business Impact:

  • Inbound Mail Halt: No new external email received
  • Partner Communication: Automated emails from vendors fail
  • Customer Impact: Incoming orders, inquiries delayed
  • Reputation: Sending servers may mark you as problematic

Common Causes of 452 4.3.1

1. Queue Database Disk Space Exhaustion (45%)

The drive containing the Transport queue database (mail.que) has less than 4GB free space. This is the most common trigger.

Threshold: Critical at less than 4GB; High at less than 8GB

2. Memory Pressure (30%)

EdgeTransport.exe or the system overall has exhausted available memory processing messages.

Threshold: Based on physical RAM; varies by server configuration

3. Submission Queue Depth (15%)

Too many messages waiting in the submission queue due to delivery failures or slow processing.

Threshold: Default 10,000 messages triggers back pressure

4. Version Store Exhaustion (7%)

The ESE database version store used for transaction management has reached capacity.

Threshold: Version buckets exceed configured maximum

5. Transaction Log Disk Full (3%)

Separate from queue database, the transaction log location may run out of space.

Threshold: Same as queue database (4GB critical)

Quick Diagnosis: Identify the Constrained Resource

📌 Version Compatibility: This guide applies to Exchange 2016, Exchange 2019, Exchange 2022. Commands may differ for other versions.

Step 1: Check Back Pressure Status
# Get real-time resource pressure status
[xml]$bp = Get-ExchangeDiagnosticInfo -Process EdgeTransport -Component ResourceThrottling
$bp.Diagnostics.Components.ResourceThrottling.ResourceTracker.ResourceMeter |
  Select-Object Name, CurrentResourceUse, PreviousResourceUse, Pressure |
  Format-Table -AutoSize

# Quick summary
$bp.Diagnostics.Components.ResourceThrottling.ResourceTracker.ResourceMeter |
  Where-Object {$_.Pressure -ne "Normal"} |
  Select-Object Name, Pressure
Step 2: Check Disk Space
# Check all drives
Get-WmiObject Win32_LogicalDisk -Filter "DriveType=3" |
  Select-Object DeviceID,
    @{Name="FreeGB";Expression={[math]::Round($_.FreeSpace/1GB,2)}},
    @{Name="TotalGB";Expression={[math]::Round($_.Size/1GB,2)}},
    @{Name="PercentFree";Expression={[math]::Round(($_.FreeSpace/$_.Size)*100,1)}} |
  Format-Table -AutoSize

# Find queue database location
Get-TransportService $env:COMPUTERNAME |
  Select-Object QueueDatabasePath, QueueDatabaseLoggingPath

Pro Tip: Back pressure critical threshold for disk space is 4GB. If your queue drive shows less than 4GB free, that's almost certainly the cause of 452 4.3.1 errors.

Step 3: Check Memory Usage
# Check EdgeTransport memory usage
Get-Process EdgeTransport | Select-Object Name,
  @{Name="MemoryGB";Expression={[math]::Round($_.WorkingSet64/1GB,2)}},
  @{Name="PrivateGB";Expression={[math]::Round($_.PrivateMemorySize64/1GB,2)}}

# Check system memory
$os = Get-WmiObject Win32_OperatingSystem
Write-Host "Free Memory: $([math]::Round($os.FreePhysicalMemory/1MB,2)) GB"2)) GB"
Write-Host "Total Memory: $([math]::Round($os.TotalVisibleMemorySize/1MB,2)) GB"2)) GB"
Step 4: Check Queue Depth
# Get queue summary
Get-Queue | Measure-Object -Property MessageCount -Sum |
  Select-Object @{Name="TotalMessages";Expression={$_.Sum}}

# Find large queues
Get-Queue | Where-Object {$_.MessageCount -gt 100} |
  Select-Object Identity, Status, MessageCount, NextHopDomain |
  Sort-Object MessageCount -Descending

Quick Fix (10 Minutes) - Emergency Resource Relief

Fix A: Free Disk Space (Most Common)

Emergency Disk Cleanup
# Find what's consuming disk space on queue drive
$queueDrive = "C:"  # Adjust to your queue drive
Get-ChildItem "$queueDrive\" -Directory |
  ForEach-Object {
    $size = (Get-ChildItem $_.FullName -Recurse -File -ErrorAction SilentlyContinue |
      Measure-Object -Property Length -Sum).Sum
    [PSCustomObject]@{
      Folder = $_.Name
      SizeGB = [math]::Round($size/1GB, 2)
    }
  } | Sort-Object SizeGB -Descending | Select-Object -First 10

# Delete old transport logs (older than 7 days)
$logPath = "-Directory |
  ForEach-Object {
    $size = (Get-ChildItem $_.FullName -Recurse -File -ErrorAction SilentlyContinue |
      Measure-Object -Property Length -Sum).Sum
    [PSCustomObject]@{
      Folder = $_.Name
      SizeGB = [math]::Round($size/1GB, 2)
    }
  } | Sort-Object SizeGB -Descending | Select-Object -First 10

# Delete old transport logs (older than 7 days)
$logPath = "C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\Logs"
Get-ChildItem $logPath -Recurse -Filter "$logPath -Recurse -Filter "*.log" |
  Where-Object {$_.LastWriteTime -lt (Get-Date).AddDays(-7)} |
  Remove-Item -Force -Verbose

# Delete old IIS logs
Get-ChildItem "$_.LastWriteTime -lt (Get-Date).AddDays(-7)} |
  Remove-Item -Force -Verbose

# Delete old IIS logs
Get-ChildItem "C:\inetpub\logs\LogFiles" -Recurse -Filter "-Filter "*.log" |
  Where-Object {$_.LastWriteTime -lt (Get-Date).AddDays(-7)} |
  Remove-Item -Force -Verbose

# Clear Windows temp files
Remove-Item "$_.LastWriteTime -lt (Get-Date).AddDays(-7)} |
  Remove-Item -Force -Verbose

# Clear Windows temp files
Remove-Item "$env:TEMP\*" -Recurse -Force -ErrorAction SilentlyContinue
Remove-Item "-Force -ErrorAction SilentlyContinue
Remove-Item "C:\Windows\Temp\*" -Recurse -Force -ErrorAction SilentlyContinue

Fix B: Restart Transport Service

Restart Transport to Release Memory
# Restart clears in-memory queues and releases memory
Restart-Service MSExchangeTransport

# Wait for service to stabilize
Start-Sleep -Seconds 30

# Check back pressure status
[xml]$bp = Get-ExchangeDiagnosticInfo -Process EdgeTransport -Component ResourceThrottling
$bp.Diagnostics.Components.ResourceThrottling.ResourceTracker.ResourceMeter |
  Where-Object {$_.Pressure -ne "Normal"}

Fix C: Clear Stuck Queues

Clear Excessive Queue Depth
# Retry all queued messages
Get-Queue | Where-Object {$_.Status -eq "Retry"} | Retry-Queue

# If queue has spam or unwanted messages, remove them
# Get-Queue | Get-Message -Filter {FromAddress -like "*spam*"} | Remove-Message -Confirm:$falseGet-Message -Filter {FromAddress -like "*spam*"} | Remove-Message -Confirm:$false

# Suspend problematic queue temporarily
# Suspend-Queue -Identity "Submission"-Identity "Submission"

# Check queue depth after cleanup
Get-Queue | Select-Object Identity, MessageCount

Fix D: Immediate Memory Relief

Stop Non-Essential Services Temporarily
# If memory pressure is critical, temporarily stop non-essential services
# WARNING: Only for emergency; restart services after resolution

# Stop POP/IMAP if not heavily used
Stop-Service MSExchangePOP3 -ErrorAction SilentlyContinue
Stop-Service MSExchangeIMAP4 -ErrorAction SilentlyContinue

# Recycle IIS application pools
iisreset /restart

# Check memory after services stopped
Get-Process | Sort-Object WorkingSet64 -Descending |
  Select-Object -First 10 Name,
    @{Name="MemoryMB";Expression={[math]::Round($_.WorkingSet64/1MB,0)}}

Detailed Solution: Permanent Resource Management

Scenario 1: Move Queue Database to Larger Drive

Relocate Queue Database
# Stop Transport service
Stop-Service MSExchangeTransport

# Create new directory on larger drive
New-Item -ItemType Directory -Path "D:\ExchangeQueues\Queue" -Force
New-Item -ItemType Directory -Path "D:\ExchangeQueues\QueueLogs" -Force

# Move existing database files
$oldPath = "C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\data\Queue"
Move-Item "$oldPath\mail.que" "D:\ExchangeQueues\Queue\"
Move-Item "$oldPath\trn*.log" "D:\ExchangeQueues\QueueLogs\"

# Update Transport configuration
Set-TransportService $env:COMPUTERNAME `
  -QueueDatabasePath "Set-TransportService $env:COMPUTERNAME `
  -QueueDatabasePath "D:\ExchangeQueues\Queue" `
  -QueueDatabaseLoggingPath "D:\ExchangeQueues\QueueLogs"

# Start Transport service
Start-Service MSExchangeTransport

Scenario 2: Adjust Back Pressure Thresholds

Pro Tip: Only raise thresholds if you've verified adequate hardware resources. Raising thresholds on an under-resourced server risks crashes instead of graceful degradation.

Modify Back Pressure Thresholds
# Edit EdgeTransport.exe.config
# Location: C:\Program Files\Microsoft\Exchange Server\V15\Bin\EdgeTransport.exe.config

# Key settings to adjust in <appSettings>:
# <add key="DatabaseDiskSpaceLowToMediumThreshold" value="8GB" />"8GB" />
# <add key="DatabaseDiskSpaceMediumToHighThreshold" value="6GB" />"6GB" />
# <add key="DatabaseDiskSpaceHighToCriticalThreshold" value="4GB" />"4GB" />

# For memory pressure:
# <add key="PrivateBytesLowToMediumThreshold" value="4GB" />"4GB" />
# <add key="PrivateBytesHighToCriticalThreshold" value="2GB" />"2GB" />

# After editing, restart Transport
Restart-Service MSExchangeTransport

Scenario 3: Add Server Resources

Evaluate Resource Needs
# Check current resource utilization
# CPU
(Get-WmiObject Win32_Processor).LoadPercentage

# Memory
$os = Get-WmiObject Win32_OperatingSystem
$usedMem = [math]::Round(($os.TotalVisibleMemorySize - $os.FreePhysicalMemory)/1MB, 2)
Write-Host "Used Memory: $usedMem GB"

# Disk IOPS (requires Performance Monitor data)
# Check for disk queue length > 2 indicates I/O bottleneck

# Recommendations:
# - Minimum 8GB RAM for Transport role
# - SSD recommended for queue database
# - Dedicated disk for queue database if high volume

Verify the Fix

Verification Commands
# 1. Confirm back pressure cleared
[xml]$bp = Get-ExchangeDiagnosticInfo -Process EdgeTransport -Component ResourceThrottling
$bp.Diagnostics.Components.ResourceThrottling.ResourceTracker.ResourceMeter |
  Select-Object Name, Pressure | Format-Table

# 2. Check no 15xxx events
Get-EventLog -LogName Application -Source MSExchangeTransport -Newest 20 |
  Where-Object {$_.EventID -match "^150"}

# 3. Test inbound SMTP
$smtp = New-Object Net.Sockets.TcpClient
$smtp.Connect("localhost", 25)
$stream = $smtp.GetStream()
$reader = New-Object System.IO.StreamReader($stream)
Write-Host $reader.ReadLine()  # Should show 220 banner, not 452452
$smtp.Close()

# 4. Verify disk space improved
Get-WmiObject Win32_LogicalDisk -Filter "DriveType=3" |
  Select-Object DeviceID, @{Name="FreeGB";Expression={[math]::Round($_.FreeSpace/1GB,2)}}

Success Indicators:

  • All resources show Pressure: Normal
  • No Event IDs 15004-15007 after fix
  • SMTP connection returns 220 banner
  • Queue database drive has more than 10GB free
  • External mail begins delivering

Prevention: Maintain Healthy Resources

1. Disk Space Monitoring

Automated Disk Space Alert
# Schedule to run every 15 minutes
$threshold = 10  # GB
$queueDrive = (Get-TransportService $env:COMPUTERNAME).QueueDatabasePath.Substring(0,2)
$freeSpace = (Get-WmiObject Win32_LogicalDisk -Filter "DeviceID='$queueDrive'").FreeSpace / 1GB

if ($freeSpace -lt $threshold) {
    Send-MailMessage -To "admin@company.com" -From "alerts@company.com" `
      -Subject "URGENT: Exchange queue drive low - $([math]::Round($freeSpace,1))GB"1))GB" `
      -Body "Queue drive $queueDrive has only $([math]::Round($freeSpace,1))GB free. Back pressure imminent."$freeSpace,1))GB free. Back pressure imminent." `
      -SmtpServer "backup-smtp.company.com"
}

2. Scheduled Log Cleanup

  • Weekly: Archive and delete old transport logs
  • Weekly: Clear old IIS logs
  • Monthly: Review and purge message tracking logs

3. Resource Planning

  • Plan 50GB minimum for queue database drive
  • 8GB RAM minimum for Transport role
  • SSD recommended for queue database
  • Monitor growth trends monthly

Back Pressure Won't Clear? Get Help Now.

If back pressure persists despite freeing resources, there may be deeper infrastructure issues - undersized hardware, storage I/O bottlenecks, or misconfigured thresholds. Our specialists diagnose and resolve complex resource issues.

Exchange Back Pressure Emergency Support

Average Response Time: 15 Minutes

Frequently Asked Questions

Error 452 4.3.1 "Insufficient system resources" is returned when Exchange activates back pressure to protect itself from resource exhaustion. This occurs when memory, disk space, or queue depth exceeds critical thresholds. It's a temporary rejection telling senders to retry later.

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