How to Troubleshoot an IIS SMTP Relay with Messages Stuck in the Queue
Question details
Users need to restore email flow when an IIS SMTP relay stops delivering messages and leaves them stuck in the Queue folder.
- Product
- IIS SMTP / Exchange Server
- Device & OS
- Windows Server
- Scenario
- Following changes to the local server or the Microsoft 365 environment, the local SMTP relay suddenly fails to route outbound messages.
- Observed behavior
- Outgoing email messages are not delivered to the target Exchange environment and remain permanently stuck in the local IIS SMTP Queue directory.
Open the Windows Event Viewer and check the System and Application logs for SMTP or Schannel connection errors, and ensure port 587 or 25 is allowed through your outbound firewall.
Enable TLS 1.2 Configuration in the Windows Registry
Use this solution if your SMTP relay fails to authenticate with Microsoft 365 due to deprecated TLS 1.0/1.1 protocols.
Microsoft 365 has retired older TLS versions. For older operating systems like Windows Server 2008 or 2012, TLS 1.2 must be explicitly enabled in the registry for the SMTP service to successfully connect and relay mail.
Ensure you do not manually enable obsolete protocols such as SSL 2.0 or SSL 3.0, as they pose security risks and are rejected by Exchange Online.
Press Windows + R, type 'regedit' into the Run dialog, and press Enter to launch the Registry Editor.
In the left pane, navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols.
Right-click the Protocols folder, select New > Key, and name it 'TLS 1.2'. Inside the TLS 1.2 key, create two new subkeys named 'Client' and 'Server'.
Inside both the Client and Server keys, create a DWORD (32-bit) Value named 'DisabledByDefault' and set its value data to 0. Create another DWORD named 'Enabled' and set its value data to 1.
Close the Registry Editor and restart your Windows Server to apply the cryptographic changes to the IIS SMTP service.
Verify IIS SMTP Connector and Microsoft 365 Smart Host Settings
Check your IIS SMTP delivery configuration to ensure emails are correctly routed to your Microsoft 365 tenant.
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Frequently Asked Questions
Why are my emails suddenly stuck in the IIS SMTP Queue folder?
This usually happens when Microsoft 365 or your target server deprecates older security protocols (like TLS 1.0 or 1.1), causing the authentication or connection to fail. The server queues the messages because it cannot establish a secure connection.
Should I enable SSL 2.0 or SSL 3.0 to fix legacy connection issues?
No. Obsolete protocols such as SSL 2.0 and SSL 3.0 are highly insecure and are entirely rejected by modern email services like Microsoft 365. You should explicitly disable them and rely solely on TLS 1.2 or higher.
Where can I get advanced help for on-premises Exchange configurations?
If you are running an on-premises Exchange server and encounter complex routing issues, it is best to post in the Microsoft Q&A Exchange Server community. The Microsoft 365 Exchange Online forums generally focus only on cloud deployments.
