The SMTP error 421 is a Microsoft Outlook error that occurs when your email account fails to connect to the SMTP serverand it shows the SMTP error code 421. If you wish to resolve the error code 421, follow the below-mentioned troubleshooting steps.
First, open the Microsoft Outlook application and click the File tab available at the top-left corner of the screen.
Navigate to the left-side panel and click the Info tab.
Under Account Information, click the Account Settings button and choose your account from the displayed list to fix SMTP error 421.
After selecting the email account, click the Change button and verify the outgoing mail server settings in the Server Information section.
Click the More Settings button available at the bottom-right side of the Change Account window.
In the Internet Email Settings window, click the Outgoing Server tab and select the “My outgoing server (SMTP) requires authentication” checkbox.
Enable the “Use server settings as my incoming server” radio button and click OK.
Open the Internet Email Settings window again and click the Advanced tab.
Configure the server port numbers for an encrypted connection.
Use the SSL or TLS authentication for both the Incoming and Outgoing servers.
Click OK to close the window and select the Test Account Settings button to verify the server settings.
Click the Finish button if everything is correct. Close the Outlook window.
Relaunch the application and check if the SMTP error 421 has been resolved.
If the problem persists, remove the existing email account and reconfigure it manually with correct server settings.
Click the Start menu and select the Settings icon, followed by Update & Security.
Select Windows Security followed by Firewall & network protection to open the Windows Security settings.
Select a network profile and toggle off the Windows Defender Firewall button.
Now, check if the SMTP error code 421 reappears by sending a test email from your email account.
If the SMTP error 421 appears again, allow the Outlook app to run through the Firewall to resolve the error.