HTTP 503: OWA/ECP Service Unavailable - Fix Guide 2025

Service Unavailable

HTTP Error 503. The service is unavailable.

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Or in some browsers:
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This page isn't working
mail.company.com is currently unable to handle this request.
HTTP ERROR 503

# Import IIS module
Import-Module WebAdministration

# Start OWA app pool
Start-WebAppPool -Name "MSExchangeOWAAppPool"

# Start ECP app pool
Start-WebAppPool -Name "MSExchangeECPAppPool"

# Verify they're running
Get-WebAppPoolState -Name "MSExchangeOWAAppPool"
Get-WebAppPoolState -Name "MSExchangeECPAppPool"

# Start all stopped Exchange pools
Get-ChildItem IIS:\AppPools | Where-Object {$_.Name -like "*Exchange*" -and $_.State -eq "Stopped"} |
  ForEach-Object { Start-WebAppPool -Name $_.Name; Write-Host "Started: $($_.Name)" }

# Stop and start to reset failure counter
Stop-WebAppPool -Name "MSExchangeOWAAppPool"
Start-Sleep -Seconds 5
Start-WebAppPool -Name "MSExchangeOWAAppPool"

# If still failing, temporarily disable rapid-fail
Set-ItemProperty IIS:\AppPools\MSExchangeOWAAppPool -Name failure.rapidFailProtection -Value $false
Start-WebAppPool -Name "MSExchangeOWAAppPool"

# Re-enable after troubleshooting
# Set-ItemProperty IIS:\AppPools\MSExchangeOWAAppPool -Name failure.rapidFailProtection -Value $true-Name failure.rapidFailProtection -Value $true

# Full IIS reset
iisreset /restart

# Wait for services to start
Start-Sleep -Seconds 30

# Verify all Exchange pools started
Get-ChildItem IIS:\AppPools | Where-Object {$_.Name -like "*Exchange*"} |
  Select-Object Name, State

# Enable Failed Request Tracing for detailed diagnostics
# In IIS Manager: Sites > Default Web Site > Failed Request Tracing Rules

# Check for memory issues
Get-Process w3wp | Select-Object Id,
  @{Name="MemoryMB";Expression={[math]::Round($_.WorkingSet64/1MB,0)}}

# Increase memory recycling threshold
Set-ItemProperty IIS:\AppPools\MSExchangeOWAAppPool -Name recycling.periodicRestart.privateMemory -Value 2000000

# Check identity permissions
$pool = Get-Item IIS:\AppPools\MSExchangeOWAAppPool
$pool.processModel.identityType
$pool.processModel.userName

# Verify app pool identity
Get-ItemProperty IIS:\AppPools\MSExchangeOWAAppPool -Name processModel |
  Select-Object identityType, userName

# Reset to default identity (LocalSystem)
Set-ItemProperty IIS:\AppPools\MSExchangeOWAAppPool -Name processModel.identityType -Value LocalSystem

# Grant permissions to Exchange folders
$exchangePath = "C:\Program Files\Microsoft\Exchange Server\V15"
$acl = Get-Acl $exchangePath
$rule = New-Object System.Security.AccessControl.FileSystemAccessRule(
    "IIS_IUSRS", "ReadAndExecute", "ContainerInherit,ObjectInherit", "None", "Allow"
)
$acl.SetAccessRule($rule)
Set-Acl $exchangePath $acl

# Restart pool
Restart-WebAppPool -Name "MSExchangeOWAAppPool"

# Ensure auto-start is enabled
Set-ItemProperty IIS:\AppPools\MSExchangeOWAAppPool -Name autoStart -Value $true
Set-ItemProperty IIS:\AppPools\MSExchangeECPAppPool -Name autoStart -Value $true

# Set start mode to AlwaysRunning
Set-ItemProperty IIS:\AppPools\MSExchangeOWAAppPool -Name startMode -Value AlwaysRunning

# Configure application initialization (preload)
Set-WebConfigurationProperty -PSPath "MACHINE/WEBROOT/APPHOST" `
  -Filter "system.applicationHost/sites/site[@name='Default Web Site']/application[@path='/owa']" `
  -Name "preloadEnabled" -Value $true

# 1. Verify app pools running
Get-WebAppPoolState -Name "MSExchangeOWAAppPool"
Get-WebAppPoolState -Name "MSExchangeECPAppPool"

# 2. Test HTTP response
$response = Invoke-WebRequest -Uri "https://localhost/owa" -UseBasicParsing -SkipCertificateCheck
Write-Host "Status: $($response.StatusCode)"

# 3. Check for worker processes
Get-Process w3wp | Select-Object Id, ProcessName

# 4. No new 503 errors in logs503 errors in logs
$iisLog = Get-ChildItem "C:\inetpub\logs\LogFiles\W3SVC1" |
  Sort-Object LastWriteTime -Descending | Select-Object -First 1
(Select-String -Path $iisLog.FullName -Pattern " 503 " | Measure-Object).Count

# Add to scheduled task (every 2 minutes)
Import-Module WebAdministration
$pools = @("MSExchangeOWAAppPool", "MSExchangeECPAppPool")

foreach ($pool in $pools) {
    if ((Get-WebAppPoolState -Name $pool).Value -ne "Started") {
        Start-WebAppPool -Name $pool
        # Alert admin
    }
}