SMTP 421

How To Fix SMTP 421 Error?

The SMTP error 421 is a temporary system problem. It usually means that there is a temporary problem occurring in the mail server, or there is a problem in the recipient’s email account.The Error 421 occurs when you are trying to send an email in Microsoft Outlook.

Before proceeding with the troubleshooting steps, take a look at the following that could be causing the SMTP 421 error.

  • Step 1: The SMTP mail server is configured incorrectly in Outlook.
  • Step 2: The built-in Windows defender can also be a cause for the SMTP error 421 .
  • Step 3: Any other third-party security application can be blocking the IP range, and that eventually blocks the email servers.
  • Step 4: The Internet service provider will automatically block the traffic flowing through the SMTP port 25, which could cause a lot of problems, including the SMTP error 421.

Follow the troubleshooting steps to help yourself resolve the SMTP 421 error.

Method 1: Change The Smtp Port Number

  • Step 1: Since the error itself is related to SMTP, you can try making changes to the SMTP settings.
  • Step 2: Open the email client (Outlook) and click the File menu located at the top left corner.
  • Step 3: Select Account settings, and a new window will open.
  • Step 4: Click the Change option from the Email tab.
  • Step 5: The Internet Email Settings pop-up window will open. Now, click the Advanced tab.
  • Step 6: In the Outgoing server field, change the port number from 25 to 465.
  • Step 7: Under the Use Following type connection drop-down list, select SSL.
  • Step 8: Click the OK button.
  • Step 9: If the port number 465 does not resolve the error, change the port number to 587 and save the changes.

Method 2: Resolve Outlook Configurations

  • Step 1: An accidental configuration in Outlook can also lead to the SMTP 421 error.
  • Step 2: So, remove your email account from Outlook ,and you can re-add your account by letting Outlook configure the settings automatically.
  • Step 3: Open the Outlook application.
  • Step 4: Click the File menu and select the Account settings option.
  • Step 5: Select the Email tab, click the email account displayed in the window and click the Remove button located just above. Then, close the window.
  • Step 6: Now, you can add the email account again to Outlook.
  • Step 7: Click the File menu.
  • Step 8: Scroll to the New option and click the Email account option.
  • Step 9: Enter the following information in the fields given below. 
  • Step 10: Enter your name:
  • Step 11: Enter your email address:
  • Step 12: Password:
  • Step 13: Re-enter Password:
  • Step 14: Note: do not click the manual setup radio button.
  • Step 15: Click the Next button.
  • Step 16: The Add Account window will show the Congratulations! Screen. It will show you the message: Your IMAP email account is successfully configured.

Method 3: Disable Proxy Server In Your Browser

If you are using a proxy server or VPN to remain anonymous while using the internet, it might also be one of the reasons for the SMTP 421 error.Follow the steps to disable the proxy settings:

Google Chrome:

  • Step 1: Click the three-dotted menu icon on the top right corner of the screen.
  • Step 2: Select the Settings option and click Advanced.
  • Step 3: Navigate to the System option under Advanced tab and select the option Open your computer proxy settings.
  • Step 4: Click the Proxy option slider to off.
  • Step 5: Click the Save button.

Mozilla Firefox:

  • Step 1: Open the Mozilla Firefox browser.
  • Step 2: Double-click the Firefox icon, select Tools, and click Options.
  • Step 3: Click Advanced and select the Network tab.
  • Step 4: In the Connections section, click the Settings option and make sure there is no proxy option selected.
  • Step 5: Close the window. Now your SMTP 421 error is resolved successfully.

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