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Outlook Error 421 Cannot Connect to SMTP | Top Five Ways to Fix It

Published By Ashwani Tiwari
Aswin Vijayan
Approved By Aswin Vijayan
Published On January 25th, 2024
Reading Time 5 Minutes Reading
Category Outlook, PST

SMTP or Simple Mail Transfer Protocol is a text-based protocol that is used for email transmissions across IPs, where you send an email to the receiver by sending instructions and receiving a response code.

But there are some Outlook users who reported that they are getting Outlook error 421 cannot connect to SMTP Server error while trying to send an email. Now, most of the user wants to know what is Outlook error 421? And why does it occurs in Outlook and also how fix the problem?

So here in this blog, we are going to discuss all the questions regarding this issue. So, let’s get started!

What is The 421 Cannot Connect SMTP Error?

Basically, Outlook 421 SMTP error hampers occur during the transmission of emails. It simply mails transfer protocol errors and may cause great inconvenience because it stops all the activities related to your email account. Once you get this SMTP error 421 you will receive messages like:

“server responded 421 cannot connect to SMTP server”

“An unknown error has occurred: Server error 421”

“Could not send your message. Error 421”

Reasons Outlook Error 421 Cannot Connect to SMTP

  • Configuration mistakes in Outlook settings are the most common reason,
  • If you are using firewalls that scan all the emails
  • If you are using a VPN connection, then there is a chance you may encounter this 421 SMTP error
  • PST file corruption is also the reason you are not able to send emails

How to Fix Error 421 Cannot Connect to SMTP Server?

Now, here in this section, we are going to discuss some techniques to fix Outlook error 421 cannot connect to SMTP. So keep reading the blog and find out the best solution for you.

#Tip 1: Modify SMTP Settings

By default, the SMTP protocol uses port 25 now if this port is blocked, then you can not able to send emails and may encounter the 421 error. So, to resolve the error, you need to change the SMTP settings. And to do this just follow the steps:

1. First, launch Outlook and go to File option

2. Now go to the Account Settings option.

3. After that select the email account that showing this error and then click the Change button.

4. Now, click More Settings option and click on the Advanced tab.

Note: If you can’t open Internet Email Settings window, press CTRL + S + ALT. In the
Send/Receive Groups window, click Edit, and select your email account. Then click Account Properties.

5. Now, in the Outgoing server (SMTP) field, set the port number to 465 and click OK.

6. Next, you need to set Outlook to automatically fetch your mail server’s settings. To do so, follow these steps:

7. In the Internet Email Settings window, open Outgoing Server settings.

8. After that, check the (SMTP) requires authentication checkbox and then select the Use the same settings as my incoming mail server option and then click OK.

#Tip 2: Remove and Re-add Email Account

Maybe there is a chance that you have wrongly configured your Outlook profile. So to fix this you can simply remove the existent email account from Outlook and re-add the same account once again but this time very carefully. And once you have re-added the account then simply one the Outlook to check the issue.

#Tip 3: Disable Firewall

As we said above that firewalls are also be the reason for this Outlook error 421 cannot connect to SMTP error. So, to fix this error check the firewall is blocking the Outlook functionality and if the Firewall is blocking certain functions of Outlook like receiving and sending of emails.

Now, if the 421 error is caused due to Firewall interference, disabling the Firewall can help fix the issue. To do that, follow these steps:

1. First, go to the Start menu and open Control Panel.

2. Now, open System and Security and then click Windows Firewall.

3. Now, click turn Windows Firewall on or off.

4. Turn off the Firewall under both private and public network settings. Then click OK.

#Tip 4. Check Other Applications

Apart from Firewall, other applications such as VPN or Antivirus may also interfere with the normal functioning of MS Outlook. Disabling such applications can help resolve the 421 cannot connect to SMTP error.

#Tip 5. Repair PST File to Fix 421 Cannot Connect to SMTP

Corrupted Outlook file is also the major reason for this error. So to fix this error you need to repair the corrupted PST file. And to repair the corrupted PST you can use the in-built utility called ScanPST.exe.

But the ScanPST.exe is only capable to repair minor level corruption from Outlook. And if the PST file is severely corrupted, then Scanpst.exe is not going to work. So In that case, you can use advanced Outlook repair software, such as Outlook Repair Tool.

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The software can easily repair severely corrupt PST files in a few steps without any hassle and data loss. It is a most powerful Outlook corruption repairing tool that also works as a recovery tool. Yes, the tool is well capable to recover deleted items from corrupted PST files.


Well, this is the time to end the blog, here we have discussed all the possible troubleshooting techniques to fix Outlook error 421 cannot connect to SMTP error. So go through the blog and get rid of the issue.