How to Fix 550 5.1.10 RESOLVER.ADR.RecipientNotFound Error in Exchange Server

First, check if the Recipient Filter Agent is installed and enabled on your Exchange server. This agent is responsible for recipient validation and is often the source of the 550 5.1.10 error.

Open Exchange Management Shell as an administrator and run:

Get-TransportAgent | Where-Object {$_.Name -like "*Recipient*"}

Look for the "Recipient Filter Agent" in the output. Note its status (Enabled/Disabled) and priority number.

If the Recipient Filter Agent isn't present, you need to install the anti-spam agents. This is required on all Mailbox servers, not just Edge Transport servers.

Run the installation script from the Exchange installation directory:

& $env:ExchangeInstallPath\Scripts\Install-AntiSpamAgents.ps1

The script will install multiple anti-spam agents including the Recipient Filter Agent. Wait for the installation to complete.

After installation, restart the Exchange Management Shell and verify the agents are installed:

Get-TransportAgent

Now enable the Recipient Filter Agent to allow configuration of recipient validation settings:

Enable-TransportAgent "Recipient Filter Agent"

Set the priority to ensure it processes messages at the correct stage:

Set-TransportAgent "Recipient Filter Agent" -Priority 5

Check the current recipient filter configuration:

Get-RecipientFilterConfig | Format-List

Pay attention to the RecipientValidationEnabled property - this is likely set to $true and causing your 550 5.1.10 errors.

Ensure your accepted domains are configured to use Address Book lookup for recipient validation. Replace 'yourdomain.com' with your actual domain name:

Get-AcceptedDomain

For each domain that should validate recipients against Active Directory:

Set-AcceptedDomain "yourdomain.com" -AddressBookEnabled $true

If you have multiple domains, repeat this for each one:

Get-AcceptedDomain | ForEach-Object { Set-AcceptedDomain $_.Name -AddressBookEnabled $true }

Verify the configuration:

Get-AcceptedDomain | Format-Table Name, AddressBookEnabled

This is the key step to resolve the 550 5.1.10 error for multi-recipient messages. When recipient validation is enabled, Exchange rejects the entire message if any recipient is invalid, even if other recipients are valid.

Disable recipient validation:

Set-RecipientFilterConfig -RecipientValidationEnabled $false

Alternatively, if you want to keep some validation but allow multi-recipient messages with mixed valid/invalid recipients, you can configure blocked recipients instead:

Set-RecipientFilterConfig -BlockedRecipients @("blocked@yourdomain.com", "spam@yourdomain.com")

View the updated configuration:

Get-RecipientFilterConfig | Format-List RecipientValidationEnabled, BlockedRecipients

Instead of completely disabling recipient validation, you can configure more granular filtering options that still allow legitimate multi-recipient messages:

Enable recipient lookup but disable validation rejection:

Set-RecipientFilterConfig -RecipientValidationEnabled $false -BlockListEnabled $true

Configure specific blocked recipients or domains:

Set-RecipientFilterConfig -BlockedRecipients @("noreply@*", "postmaster@spam.com")

Set up recipient lookup timeout to prevent delays:

Set-RecipientFilterConfig -RecipientValidationEnabled $true -BlockListEnabled $false

For organizations that need validation but want to handle multi-recipient scenarios gracefully, consider enabling validation with custom rejection responses:

Set-RecipientFilterConfig -RecipientValidationEnabled $true -RejectionResponse "Recipient not found in directory"

Transport agent configuration changes require restarting the Microsoft Exchange Transport service to take effect:

Restart-Service MSExchangeTransport -Force

Monitor the service restart:

Get-Service MSExchangeTransport

Wait for the service to show "Running" status. Also restart the Front End Transport service if you're running Exchange 2016/2019:

Restart-Service MSExchangeFrontEndTransport -Force

Check the transport service logs for any errors during startup:

Get-EventLog -LogName Application -Source "MSExchange*" -Newest 10

Create a test scenario to verify the 550 5.1.10 error is resolved. Send a test email to multiple recipients including both valid and invalid addresses.

Use PowerShell to send a test message:

Send-MailMessage -From "test@yourdomain.com" -To "valid@yourdomain.com", "invalid@yourdomain.com" -Subject "Multi-recipient test" -Body "Testing recipient validation fix" -SmtpServer "your-exchange-server.yourdomain.com"

Monitor the message tracking logs:

Get-MessageTrackingLog -Start (Get-Date).AddMinutes(-10) -Sender "test@yourdomain.com" | Format-Table Timestamp, EventId, Recipients, MessageSubject

Check for delivery to valid recipients and proper handling of invalid ones:

Get-Queue | Where-Object {$_.MessageCount -gt 0}

After implementing the fix, monitor your Exchange server for any unintended consequences and fine-tune the configuration as needed.

Set up monitoring for NDR generation:

Get-MessageTrackingLog -EventId "FAIL" -Start (Get-Date).AddHours(-1) | Group-Object Recipients | Sort-Object Count -Descending

Review recipient filter statistics:

Get-TransportServer | Get-MessageTrackingLog -EventId "AGENTINFO" -Start (Get-Date).AddHours(-24) | Where-Object {$_.Source -eq "AGENT"} | Group-Object EventId

If you notice increased spam or backscatter, consider re-enabling validation with exceptions:

Set-RecipientFilterConfig -RecipientValidationEnabled $true -BlockedRecipients @("common-typos@yourdomain.com")

Create a scheduled task to regularly review recipient filter effectiveness:

$ScriptBlock = { Get-RecipientFilterConfig | Out-File "C:\Logs\RecipientFilter-$(Get-Date -Format 'yyyyMMdd').log" }
Register-ScheduledJob -Name "RecipientFilterMonitoring" -ScriptBlock $ScriptBlock -Trigger (New-JobTrigger -Daily -At "06:00")