4 Methods To Fix Outlook SMTP Server Error Code 421

One SMTP error is all it takes to stop important emails from reaching their destination. But resolving the “421 Cannot Connect to SMTP Server” error is often simpler than you might expect. If you are experiencing the same issue, then the article is for you.

The error message can appear in different ways:

Stay with us to fix Outlook SMTP Server Error Code 421 by getting all the required information.

List Of Issues You May Face When 421 SMTP Error Appears [After-Effects]

You can’t perform the following actions while facing the Outlook 421 SMTP error:

• Outlook cannot send emails.
• Emails stuck remain in the Outbox.
• Password prompts keep appearing.
• Connection timeout reported by Outlook.
• Outlook disconnects while sending emails.
• The application displays the error repeatedly.
• Emails with large attachments consistently fail.
• Send/Receive operation takes longer than usual.

Top Reasons: Why “421 Cannot Connect to SMTP Server” Error Occurs?

Here is the list of reasons that might be responsible for the Outlook SMTP error code:

• Mail server downtime.
• Outlook profile corruption.
• Multiple SMTP connections.
• Disable SMTP authentication.
• Internet connectivity problems.
• Wrong SMTP port configuration.
• Incorrect SMTP server settings.
• Damage or corruption in Outlook data files.
• An antivirus or firewall system blocks Outlook.

How to Fix Outlook SMTP Server Error Code 421 Manually?

We have tried several solutions to resolve the SMTP error, but only the provided ones worked for us. Try them individually to find the most appropriate one for yourself.

Method 1. Verify the Configured SMTP Server Settings

Incorrect SMTP server settings are one of the primary reasons behind the 421 cannot connect to SMTP Server error. Here’s how to check it:
Step 1. Open Outlook and go to the File menu.
Step 2. Expand Account Settings and select Account Settings.
Step 3. Choose the email account and proceed to Change.
Step 4. Check that the SMTP Server (Outgoing) matches your email provider.

Examples of the most frequent providers are as follows:

Step 5. Then, go to More Settings and click on Advanced tab.
Step 6. Verify the entered SMTP ports. Here are the common ports:

Step 7. Confirm the encryption methods properly.

Step 8. Once done, click OK to save settings. Then, proceed to Next.
Step 9. Send a test mail to check whether the issue is resolved or not.

Method 2. Temporary Disable Firewall & Antivirus Software

Sometimes, firewall and antivirus programs can block the connection of Outlook to SMTP Servers. So, disabling them may fix 421 cannot connect to SMTP server error. Here’s how:

Note: Re-enable the security software immediately after the testing is done.

Step 1. Close all the Outlook instances.
Step 2. Disable the antivirus email scanning or protection feature.
Step 3. Access the Control Panel > Windows Firewall.
Step 4. Click on the Turn Windows Firewall on or off option to proceed.
Step 5. Turn off Windows Firewall for your current network and hit OK.
Step 6. Restart Outlook and try to send a test mail.

Method 3. Recreate the Outlook Profile

A new Outlook profile creation can often solve Outlook SMTP server error code 421. So, follow the given steps to do the same:
Step 1. Close the Outlook application.
Step 2. Access the Control Panel and proceed to Mail (Microsoft Outlook).
Step 3. Click on Show Profiles to proceed.
Step 4. Add a new profile by providing a name. Then, hit OK.
Step 5. Set up the same email account by following the on-screen instructions.
Step 6. Select Always use this profile as default.
Step 7. Tap on Apply to save changes and click OK.

Method 4. Repair Outlook Data File With the Inbox Repair Tool

A damaged or corrupted Outlook data file can also cause the Outlook SMTP server error code 421. You can fix it by using the Inbox Repair Tool (SCANPST.exe). Here are the steps:
Step 1. Close the Outlook application if opened.
Step 2. Locate and Open the SCANPST tool on the system.
Step 3. Browse to add the data file and Start the procedure.
Step 4. Click on the Repair option once scanning is done.
Step 5. After the process completion, tap on OK.

Note: You can only fix minor corruption or file inconsistencies via the SCANPST tool. For severely damaged or corrupted files, use the Recoveryfix Outlook PST Repair software. It repairs PST files quickly and restores the contained data in the original structure.

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Final Thoughts

Here, we have presented different ways to fix Outlook SMTP server error code 421. Try either manual or automated solutions to restore Outlook functionality. Native ones help you resolve minor configuration issues or inconsistencies, while the recommended one repairs the highly damaged files easily. The software is also able to fix other PST file issues, such as when the message interface has returned an unknown error.