Error 421 cannot connect to SMTP server in Outlook [EXPERT FIX]


Ivan Jenic
by Ivan Jenic
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The server responded: 421 cannot connect to SMTP server error is basically associated with Microsoft Outlook accounts, and it is commonly encountered while trying to send an email via the platform. While there are numerous factors that could cause this error, we’ll be looking into some of the most notorious ones, with emphasis on how to resolve the issue.

What to do if Outlook won’t connect to SMTP server? Firstly, reconfigure SMTP settings to fix the problem. The common reason is misconfigured SMTP (Simple Mail Transfer Protocol). Alternatively, check VPN and antivirus settings. If necessary, whitelist Outlook in order to avoid antivirus firewall blocking it.

Read about our solutions in detail below.

How to fix Error 421 cannot connect to SMTP server in Outlook

  1. Reconfigure SMTP settings
  2. Check Virtual Private Network (VPN) interference in Outlook
  3. Check for Antivirus interference in Outlook

1. Reconfigure SMTP settings

When this error is faced in MS Outlook, the most probable cause, as stated earlier, is the inappropriate configuration of the SMTP (email transmitting server) settings. Hence, it is the first troubleshooting technique we’ll be recommending in this tutorial.

To reconfigure your SMTP settings in MS Outlook, follow the steps outlined below:

  1. Launch MS Outlook.
  2. Locate and click on File.outlook file
  3. Select Account Settings.outlook account settings
  4. Click on your account and then click on Change.outlook account settings change
  5. Locate “Outgoing mail server (SMTP)” and enter the address (as provided by your web host).outlook Outgoing mail server (SMTP)
  6. Locate and click on More Settings.
  7. Navigate to the Outgoing Server tab
  8. Check the My outgoing server (SMTP) requires authentication
  9. Select the Use the same settings as my incoming mail serveroutlook Outgoing mail server (SMTP) use same settings as my incoming mail server
  10. Go back to the More Settings window and click on the Advanced tab
  11. Under the Advanced tab, locate and change your port number, as provided by your Internet Service Provider.error 421 cannot connect to smtp server outlook
  12. Save settings and close program.
  13. Check if the error is fixed: open Outlook and try sending an email.

If the problem persists, you can try the next fix.

Note: The above-outlined steps can be applied on Microsoft Outlook 2016 (and above).


2. Check Virtual Private Network (VPN) interference in Outlook

VPN is basically used to mask locations and gain access to geo-blocked/geo-restricted sites and contents, especially when in restricted areas. To some extent, this interferes with your browsing behavior, including your access and transmission of emails in Outlook.

While VPNs have little or no direct influence on email transmission, there have been reported cases of VPN services interfering with theInternet-based programs like Outlook. Hence, you can check to see if the error is caused due to interference from your VPN.

To check if the VPN is the one causing the error, simply disable your VPN service; then, you can try sending an email. If the email goes through without an issue, the VPN is probably the cause of the error. Otherwise, you can try and check if there are other interfering parties.


3. Check for Antivirus interference in Outlook

Most advanced antivirus and other security programs are equipped to run checks on browsing activities, and where required, place restrictions. In rare cases, this could be the issue when The server responded: 421 cannot connect to SMTP server error occurs.

You can check, to ascertain the culpability of a security program on your system, by disabling it, and then try sending an email. If the email goes through without a hitch, the security program is obviously culpable.

To resolve this, you can reconfigure your AV or security program to grant permissions or exclude the email SMTP server from all advanced security checks and restrictions. If you have no idea how to go about this, you can easily disable the firewall or AV anytime you want to send an email.

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