Wondershare Repairit
Email Repair

Perfectly repair and recover corrupted Outlook PST & OST files.

  • Fix all Outlook Email issues on Windows and Mac in various scenarios.
  • One trick to fix corrupted PST & OST files in Outlook.
  • Restore Outlook Emails deleted for a long time and extract important Email messages, folders, calendars, etc.
  • Perfectly compatible with Windows & Mac devices.
email repair

Approach to Fix Outlook 421 Cannot Connect to SMTP Server Error?

SMTP (Simple Mail Transfer Protocol) is a text-based protocol that handles email transmissions across IPs, where you send an email to the receiver by sending instructions and receiving a response code. Often there is an "Outlook 421 Cannot Connect to SMTP Server" Error when trying to send an email. What is Outlook error 421? And why does your server respond 421 cannot connect to SMTP? And How to fix the problem. Let's get down to it together.

Also, before troubleshooting, make sure the Outlook program hasn't crashed or corrupted.

What is Error 421?

Outlook Error 421 is a common critical SMTP error that hampers transmissions of emails. The error is to be resolved on time or may cause great inconvenience as it stops and locks all the activities related to your email account. This error occurs for temporary problems while sending and receiving emails on the server. It may also occur once you have crossed a restricted limit on the email account.

SMTP error 421 may display in formats like:

  • "server responded 421 cannot connect to SMTP server"
  • "An unknown error has occurred: Server error 421"
  • "Could not send your message. Error 421"

Common reasons for this critical Outlook error 421 are:

  • Configuration mistakes in Outlook settings are the most common reason, but this happens only when there's a recent or new configuration of an email account.
  • Issues related to Firewalls don't come up with built-in Windows firewalls. However, 3rd party firewalls and anti-viruses block connections to some IP ranges to protect users from spam
  • Automated blocked traffic on port 25 or 26 to prevent users from spam networks
  • VPN connection interference also triggers the error 421
  • PST file corruption
  • Virus interference

How To Fix "421 Cannot Connect To SMTP Server in Outlook" Issue?

First of all, Let's start with a video we curated for you, which covers the most common reasons that cause Outlook cannot connect to SMTP server, and how to fix them accordingly.

Hopefully the video has solved your generic audio service detected issue, but if not, no worries, below are some other solutions we prepared for you:

1 Reconfigure SMTP settings

Outlook error 421 can be fixed by reconfiguring the SMTP settings. So firstly, we will try to do this.

Open the Mail application and opt for the "Settings" icon. Under settings, click on the "Accounts".

reconfigure smtp settings 1

Now, select the email account you want to edit.

reconfigure smtp settings 2

Select the "Change mailbox sync settings" option.

reconfigure smtp settings 3

Next, scroll down and select "Advanced mailbox settings".

reconfigure smtp settings 4

Now replace "Outgoing (SMTP) email server" information by adding ":26:1" after the outgoing mail server (the address will look like smtp.example.com:26:1) and then check all the below boxes to yes. (SSL connection keeps the data encrypted and safe from malicious users)

reconfigure smtp settings 5

Select "Done" and "Save" and you are complete!

2 Remove then add the email account

A misspelled server name or a wrong port setting due to recent configuration in Outlook also may create error 421. It can be fixed by removing and then adding the account again.

To Remove an Email Account

Select "File" from main Outlook window

remove add email account 1

  • From the "Account Settings" select the account you want to delete and then select Remove. Now, confirm to "Yes"
  • After removing the account, add it again.>
  • Select "Settings" and then choose "Manage Accounts"

remove add email account 2

Select "Add account".

remove add email account 3

Choose the account you would want to add.

remove add email account 4

Enter your credentials and Sign in. Click "Done"

3 Change the SMTP port number

Sometimes wrong SMTP configuration can be the reason for the Outlook error 421. Hence, changing the SMTP port can fix the problem.

Open the Outlook app, select the "File" menu.

change smtp port number 1

Now, tap "Account Settings"

change smtp port number 2

Now, choose your account and select "Change".

change smtp port number 3

Look for the "Outgoing mail server (SMTP)" and key in the address provided by the web host.

change smtp port number 4

Click "More Settings" > Outgoing Server" > check "My outgoing server (SMTP) requires authentication" > "Use the same settings as my incoming mail server".

change smtp port number 5

  • Return to "More Settings" > click "Advanced".
  • Change port number and save settings.

change smtp port number 6

4 Check Virtual Private Network (VPN) interference in Outlook

Sometimes using a VPN solution to secure your web traffic might prevent Outlook from communicating with other servers. Hence there's a need to check if that's causing error 421, simply by disabling the VPN network connection and then sending an email through Outlook. If the email is sent after disabling, contact the VPN network support and ask to whitelist the email server

Disable the VPN connection through the following.

  • Open the "Settings" on your desktop.
  • Choose "Network & Internet", then opt for "VPN".

remove vpn

  • On the right side, select your VPN connection.
  • Now, click on the "Remove" button and confirm it. You're done.


5 Try to disable your Firewall

Firewalls are must-have security barriers that help keep our computers safe and protected from malicious hacks of files and resources. Firewalls are good but sometimes 3rd party firewall blocks some IP ranges and hence might cause Outlook error 421. To fix the error try disabling Firewall.

Disable the Firewall by following the steps:

From the "Start" button of your PC, select the "System and Security".

disable firewall 2

Select the bubble to turn the Windows Firewall off.

disable firewall 3

Choose "OK" to save changes.

Repair PST file

A Personal Storage Table (.pst) or Outlook PST files contains all the important data and information related to users account like emails, contacts, attachments, calendar, etc. With time the size of PST files increases and heavy PST file is a major cause of its corruption. However, Wondershare Repairit for Email is a powerful tool to repair the damaged PST file of any size, recovering with high precision.

  • Repair damaged PST & OST Files in Outlook in all Email corruption scenarios, such as hard disk crashes & bad sectors, virus attacks & tojan horse, outlook corruption & malfunctions, improper shutdown procedure, etc.

  • Restore Outlook Emails deleted for a long time and extract the Email messages, folders, calendars, etc. within seconds.

  • Repair corrupted Emails and restore deleted Emails with a 100% success rate.

  • Repair Emails in bathes without size limits and support to preview all of them before saving.

  • Compatible with Windows 7/8/8.1/10/11, Windows Server 2012/2016/2019, and MacOS X 10.11~ MacOS 12 (Monterey).

The Repairit software repairs the data and restores them in the original format and also, in a new usable PST file. The new PST file can also be imported into Outlook giving access to all the email items.

To repair the PST file using the software Repairit for Email follow the methods below:

Step 1: Add PST File

Install Repairit for Email on your PC, and click "+Select File" to add the corrupt PST file.

add corrupt PST file

Step 2: Repair PST File

After selecting the file, now click on the 'Repair' option to start repairing your corrupted PST file.

choose PST repair

It will start showing the scanning process, once it is complete, Repairit will show you all the recoverable folders. Now, select the content you want to view and it will display in the same pane.

start PST repair

Step 3: Save Repaired File

Now, preview and save the recovered files, click on the "Save" option and it will allow you to save the file on you computer or Mac.

preview and save repaired pst file

Once you've chosen the location and format of the file, click "OK" to save. You're done with repairing of PST file.

The Bottom Line

Follow these methods to fix the 'Outlook 421 Cannot Connect to SMTP Server Error'. Wondershare Repairit for Email is effective software that helps recover PST files and restore all the Outlook data. This includes deleted and missing emails that were lost due to PST file corruption. It is recommended software which recovers mailboxes from password-protected PST files and also Deleted Outlook Mailbox Items with fast and easy recovery.