How to Fix Server Responded: 421 Cannot Connect to SMT / SMTP server?

SMTP stands for Simple Mail Transfer Protocol that is designed for electronic mail transmission. As per reports, lately, some of the MS Outlook users have claimed that while sending an email their server responded with a message stating, “Server Responded: 421 Cannot Connect to SMT / SMTP server”. This error is only associated with the MS Outlook account despite being it of any version.

As we know the need for sending and receiving emails in our daily life, including personal and professional matters. Knowing this, one cannot afford to experience a similar error regularly; hence it becomes crucial to get rid of it as soon as possible. In this article, we have incorporated 4 tried and tested fixes that will help you get rid of the said error.

FIXES for “Server Responded: 421 Cannot Connect to SMT / SMTP server” error.

The SMT/ SMTP error can get triggered due to several reasons. Below here, are all the possible causes and their relatable fixes.

FIX 1: Reconfigure the SMTP settings:

If the ” Server Responded: 421 Cannot Connect to SMT / SMTP server” error occurs in MS outlook, the reason behind it would possibly be the wrong SMTP configuration settings. Hence, reconfiguring the STMPS settings would help here to resolve the error issue. For doing so,

NOTE: This procedure is only applicable to MS Outlook 2016 and above.

  • Firstly, launch MS Outlook.
  • Now on the top left corner, click on File and then select Account settings.
  • On the Account setting window, click on your Account and then click on Change.
  • On the Change Account window, locate Outgoing mail server (SMTP) and type the address provided by your web host in the adjoining input box.
  • Now click on More settings. (on the bottom right).
  • On the next prompt, click on the Outgoing Server tab from the vertical menu.
  • Now tick the checkbox located before the option My outgoing server (SMTP) requires authentication.
  • Also, click on the circle icon located before the option Use the same settings as my incoming mail server.
  • Now click on Advanced tab from the same vertical menu located above.
  • In the Advanced section menu, change the Port number with the one that is provided by your Internet Service Provider.
  • Finally, click on OK to save the settings.
  • Close the tab and open MS outlook to send an email and verify if the error is resolved or persist.

FIX 2: Remove and then again add the email account:

The ” Server Responded: 421 Cannot Connect to SMT / SMTP server” error might also be a result of a misspelled server name or wrong port setting due to some current configuration setting in MS outlook. If this is the case, then removing and adding the email again might help in resolving the issue. For doing so,

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For removing the email account:

  • Launch MS Outlook.
  • On the Outlook window, navigate to the top left corner and select File.
  • Now go to Account setting, click on the Account you want to delete and then select Remove.
  • Lastly, click on Yes to confirm.

For adding the email account:

  • On the MS Outlook window, go to Settings and select the option Manage Accounts.
  • Now click on Add account and from the next prompt choose the Account you want to add.
  • Here, enter the credentials and then click on Sign in.
  • Lastly, click on Done.

FIX 3: Check the VPN interference in Outlook:

VPN (Virtual Private Network) forms a private network from a public internet connection for providing users anonymity and online privacy. But at the time, the VPN solution can block Outlook and refrain it from communicating with other network servers.

To mark out this possibility, users must disable their VPN and then try to send an email through MS outlook. If the email is sent smoothly, then it is evident that the VPN is interfering the Outlook.

For disabling the VPN server, follow the steps given below:

  • Go to the search taskbar and open the Settings window.network and security
  • Click on Network & Settings, then go to the left pane and select the option VPN.
  • From the right-pane menu, locate and select your VPN connection and then click on the Remove button.
  • Your VPN connection is now disabled.

Hereafter, if you find that the VPN is interfering the Outlook then consider connecting with your VPN server support team and ask them to Whitelist the email server.

FIX 4: Disable the Firewall interference:

A firewall is a network security system that keeps your files and resources safe by forming a barrier between safe and unsafe network. Though some third-party firewalls can block some IP ranges which in result might trigger the ” Server Responded: 421 Cannot Connect to SMT / SMTP server” error. To rule out this possibility, disabling the Firewall might help. For doing so,

  • Open Control Panel and select System & Security.Windows Defender Firewall
  • On the next window, locate and select Windows Defender Firewall.
  • Now customize the settings by clicking on the circle icon located before the option Turn off Windows Defender Firewall (not recommended).
  • Lastly, click on OK to save the changes.
  • Once done, now try sending email through Outlook and if it proceeds successfully then it proves that your third-party Firewall was causing blockage on the way.
  • Hereafter, start looking for your third-party firewall settings and try to add the email server to its Exclusion list. The process for adding the email server will depend upon the third-party Firewall you are using.

The SMTP server is a crucial element used in our day to day life for sending emails over the internet. And when the ‘Server Responded: 421 cannot connect with the SMTP server’ error arises, then the user might be refrained from sending emails across the internet. Under such circumstances, the fixes, as mentioned above, will help.

Hopefully, through this article, you have got useful and relevant information. Also, as a disabling Firewall and any third-party security software for long can prove to be a threat to your system, we advise you to refrain from doing so. If you have any queries or feedback, please write down the comment in the below comment box.

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