How To Fix Outlook 421 Cannot Connect To SMTP Server Error

Microsoft Outlook is an email application that is available on the Windows platform. It allows anyone to manage their emails and includes other features such as a calendar, task manager, contact manager, note taking, journal, and web browsing. Although this is a solid performing application there are instances when certain issues can occur which we will be addressing today. In this latest installment of our troubleshooting series we will tackle the Outlook 421 cannot connect to SMTP server error.

How To Fix 421 Cannot Connect To SMTP Server Outlook

Before performing any of the recommended troubleshooting steps listed below it’s best to restart your computer first. This action refreshes the operating system and remove any corrupt temporary data that could be causing the problem.

Remove then add the email account

If this issue occurred right after adding an email account then there’s a possibility that this is caused by a wrong configuration. Try removing the account then adding it again. Make sure that you have the necessary information required to add the email account. 

  • Open Outlook and go to File > Account Settings, select the account in question and click the Remove button.
  • Click the New button, select Email account and insert your credentials again.
  • Click Next and wait for the settings to be configured and the test email message to be sent.

Check if the Outlook 421 cannot connect to SMTP server error still occurs.


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Change the SMTP port number 

The SMTP or Simple Mail Transfer Protocol usually uses port 25. If this port is being blocked then there’s a chance that this problem will occur. You can try changing this to port 465 then check if the issue still occurs. Take note that this is only applicable if your outgoing server (SMTP) connection is configured through SSL.

  • Open Outlook then go to File > Account Settings, select the email account with the problem and hit the Change button.
  • Go to More Settings and select the Advanced tab. Then, type 465 in the box near Outgoing server (SMTP) and make sure the type of encrypted connection is set to SSL.

Check if the Outlook 421 cannot connect to SMTP server error still occurs.

Try to disable your Firewall

A firewall works by regulating the internet traffic on your computer. Sometimes though it can block certain functions of Outlook such as the sending or receiving of emails. To check if this is what’s causing the problem you should try to disable your firewall.

  • Select the Start button > Settings > Update & Security  > Windows Security and then Firewall & network protection.  Open Windows Security settings
  • Select a Network Profile.
  • Under Windows Defender Firewall, switch the setting to OFF. Turning off Windows Defender Firewall could make your device (and network, if you have one) more vulnerable to unauthorized access.

Check if the Outlook 421 cannot connect to SMTP server error still occurs.

If the problem does not occur then you should allow Outlook through the Firewall.

Turn on your Firewall

  • Select the Start  button > Settings > Update & Security  > Windows Security and then Firewall & network protection. Open Windows Security settings
  • Select a network profile.
  • Under Windows Defender Firewall, switch the setting to On. 

Allow Outlook through the Firewall

  • Select the Start button > Settings > Update & Security  > Windows Security and then Firewall & network protection.  Open Windows Security settings
  • Select Allow an app through firewall.
  • Select the app you want to allow through.
  • Select OK.