Blog

If you are seeing the error “421 Cannot Connect to SMTP Server” when trying to send emails from Outlook on Windows 11, it means your email client is having trouble reaching the outgoing mail server.

This issue prevents you from sending emails and can be caused by several common problems.

Don’t worry—fixing this error usually involves checking some simple settings and connections.

In this article, we will walk you through easy steps to resolve the error and get your Outlook email working again.

Quick Note Before You Start

• Make sure your internet connection is stable and working.
• Confirm your email account settings, especially the SMTP server address and port number.
• Check if your email provider’s SMTP server is currently online and not down for maintenance.
• Have your email account username and password ready if re-authentication is needed.

Step 1: Check Your Internet Connection

Before diving into Outlook settings, ensure your Windows 11 device is properly connected to the internet. Without internet access, Outlook cannot communicate with any mail servers.

Try opening a website in your browser or running a quick speed test. If there is no internet, troubleshoot your network or restart your router.

Step 2: Verify SMTP Server Settings in Outlook

Incorrect SMTP server information is one of the most common causes of the 421 error. To check and update these settings:

1. Open Outlook.
2. Click File in the top-left corner.
3. Choose Account Settings and then click Account Settings… again.
4. Select your email account from the list and click Change.
5. Click More Settings in the bottom-right corner.
6. Go to the Outgoing Server tab. Make sure My outgoing server (SMTP) requires authentication is checked.
7. Switch to the Advanced tab.
8. Verify the SMTP server port number matches your provider’s recommended port. Common ports are 587 or 465 for SSL/TLS.
9. Ensure the encryption method is set correctly (usually STARTTLS or SSL/TLS).
10. Click OK, then Next, and finally Finish.

Using the correct SMTP server settings ensures Outlook can communicate securely and successfully with your mail server.

Step 3: Temporarily Disable Firewall or Antivirus

Sometimes, firewall or antivirus software blocks the connection to the SMTP server, causing the 421 error. To test if this is the cause:

1. Temporarily disable your firewall or antivirus software.
2. Try sending an email again from Outlook.
3. If it works, you know the security software is blocking the connection.
4. Re-enable your firewall/antivirus and add Outlook or the SMTP server to the software’s exceptions or trusted list.

This step is important because these security tools can mistakenly block legitimate outgoing email traffic.

Step 4: Restart Outlook and Your Computer

Sometimes, simply restarting Outlook or your entire computer can clear temporary glitches affecting SMTP connectivity.

Close Outlook completely, restart your computer, and then open Outlook again to test the connection.

Step 5: Check for Windows 11 and Outlook Updates

Running outdated versions of Windows or Outlook can cause compatibility issues. Check for updates:

• Open Settings in Windows 11.
• Go to Windows Update and click Check for updates.
• Install any available updates and restart if prompted.
• Also, open Outlook and check under File > Office Account > Update Options to update Outlook.

Keeping your software up to date helps avoid bugs and improves security.

Alternative Methods and Advanced Options

Use an Alternative SMTP Port

If your current SMTP port is blocked by your ISP or network, try switching to a different common port:

• 587 (with STARTTLS encryption) is widely recommended.
• 465 (with SSL/TLS encryption) is an alternative.

Change the port in Outlook’s SMTP settings (see Step 2) and test sending again.

Test SMTP Server Connection Using Telnet

For advanced users, you can check if your computer can reach the SMTP server using the Telnet tool:

telnet smtp.yourprovider.com 587

Replace smtp.yourprovider.com and 587 with your SMTP server and port. If the connection opens, the server is reachable.

Frequently Asked Questions (FAQs)

Why do I get error 421 in Outlook?

This error usually means Outlook cannot connect to the SMTP server. Causes include wrong server settings, network issues, or security software blocking the connection.

Can I still receive emails if SMTP is not working?

Yes, receiving emails uses the incoming mail server (IMAP or POP3), so sending is affected separately from receiving.

Is port 25 the right SMTP port?

Port 25 is often blocked by ISPs to prevent spam. It’s better to use port 587 or 465 for sending emails securely.

Do I need to enable SMTP authentication?

Yes, most email providers require SMTP authentication to verify your account before sending emails.

What if my antivirus blocks Outlook’s SMTP?

You should add Outlook as a trusted app or whitelist the SMTP server address in your antivirus settings.

When Nothing Works

If you have tried all the above steps and still get the “421 Cannot Connect to SMTP Server” error, consider these final options:

• Contact your email service provider’s support to check if there are server-side issues.
• Try setting up your email account on a different email client to isolate the problem.
• Use Microsoft’s official support site for Outlook troubleshooting: Microsoft Outlook Support.
• Consider temporarily switching to a different network (for example, mobile hotspot) to rule out ISP restrictions.

Conclusion

The “421 Cannot Connect to SMTP Server” error in Outlook on Windows 11 usually results from incorrect SMTP settings, network problems, or security software interference.

By carefully checking your internet connection, verifying SMTP settings, and managing your firewall or antivirus, you can resolve this issue in most cases.

Remember to keep your software updated and consult your email provider if problems persist. With these steps, you should be able to send emails smoothly again.