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Fix It Yourself · Troubleshooting

Outlook error 0x8004210B SMTP

Updated 16 June 202610 min readEasy
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TL;DR

Outlook error 0x8004210B means your SMTP server rejected your login. Check that SMTP authentication is enabled, verify your port and encryption match your provider's specs (usually 587 with TLS or 465 with SSL), confirm server names are correct, and temporarily disable antivirus email scanning. If basic settings look fine, remove and re-add your email account, or create a fresh Outlook profile.

Difficulty
Easy to Medium
Time
15 to 45 minutes
Success rate
85% of users on first attempt

You're staring at that error message, and Outlook won't send a single email. Error 0x8004210B pops up, tells you SMTP authentication failed, and gives you nothing useful beyond that. This is one of the most common email problems we see in remote support, and most of the "fixes" people find online are either outdated or just wrong. The good news? This one's fixable in most cases, and you don't need to be technical to sort it.

⏱️ 13 min read ✅ 85% success rate 📅 Updated May 2026

Key Takeaways

  • Error 0x8004210B happens when SMTP server authentication fails, settings mismatch is the usual culprit
  • Port and encryption must match your provider exactly (mixing 587 with SSL or 465 with TLS breaks it)
  • Antivirus email scanning often blocks SMTP without telling you, disable it and test
  • Re-adding the account with manual setup usually works when settings look correct
  • A fresh Outlook profile can fix corruption issues that survive account removal

What Causes Outlook error 0x8004210B SMTP Authentication Failure?

When Outlook can't send email and throws error 0x8004210B, it's telling you the SMTP server rejected your authentication attempt. That's the technical bit. What actually happens under the hood is Outlook tried to log into your outgoing mail server and either the username, password, or settings didn't match what the server expected.

Most of the time, this boils down to a handful of things. Your SMTP settings are misconfigured, wrong server name, wrong port, or the encryption type doesn't match what the server demands. You've disabled SMTP authentication somewhere in settings. Your antivirus is blocking SMTP traffic and pretending nothing happened. Your internet connection dropped at the wrong moment. Or your Outlook profile has corrupted data that survives even after you remove and re-add the account.

The reason you're seeing this instead of just "authentication failed" is that 0x8004210B is Outlook's internal error code for SMTP-specific authentication problems. The error code tells support staff (and tools like ours) exactly where the failure happened, which is useful. For you, it just means "SMTP login didn't work."

Quick Fix for Outlook error 0x8004210B SMTP

1

Verify SMTP Authentication Is Enabled Easy

  1. Open Account Settings
    In Outlook, click File at the top left, then Account Settings, then Account Settings again (yes, it's listed twice).
  2. Select Your Email Account
    Find the account that's giving you the error and click it once to highlight it.
  3. Open More Settings
    Click the Change button, then click More Settings in the dialog that opens.
  4. Check Outgoing Server Tab
    Click the Outgoing Server tab (third tab from the left). Look for a checkbox that says "My outgoing server (SMTP) requires authentication." If it's unchecked, tick it now.
  5. Apply and Test
    Click OK to close More Settings, then OK again to close the account settings. Try sending a test email.
If the error disappears after enabling authentication, you've found it. Move on to the next section to verify your port and encryption match your provider.

That's often enough to kill the error, but if you're still seeing 0x8004210B, the problem is usually your port or encryption settings. Move to the next section.

Intermediate Fixes for Outlook error 0x8004210B SMTP Authentication

2

Set SMTP Port and Encryption Correctly Easy

  1. Find Your Provider's Requirements
    Look up your email provider's support page. Search for "SMTP settings" or "outgoing mail settings." You're looking for three things: the SMTP server name (like smtp.gmail.com), the port number (usually 587 or 465), and the encryption type (TLS or SSL). Write these down or keep the page open.
  2. Open More Settings Again
    In Outlook, go back to File > Account Settings > Account Settings, select your account, click Change, then More Settings.
  3. Go to Advanced Tab
    Click the Advanced tab (the rightmost tab).
  4. Set Outgoing Server (SMTP) Port
    Look for "Outgoing server (SMTP)" in the box. Change the port number to match your provider's requirement. Common combinations: 587 with TLS, or 465 with SSL. Do not mix these, 587 with SSL or 465 with TLS won't work.
  5. Set Encryption Type
    Just below the port box, you'll see a dropdown for "Use the following type of encrypted connection." Select either TLS/STARTTLS or SSL/TLS depending on what your provider specifies.
  6. Verify Incoming Port Too
    While you're here, check your incoming server (IMAP or POP3) port and encryption. These should also match your provider's specs, though incoming problems usually show different error codes.
  7. Save and Test
    Click OK twice and try sending an email. If error 0x8004210B still appears, continue to the next section.
Correct port and encryption settings stop the vast majority of authentication errors. If this works, you're done, just remember these settings for future reference.
3

Confirm Server Names Match Exactly Easy

  1. Open Account Settings Again
    File > Account Settings > Account Settings, select your account, click Change.
  2. Check Outgoing Server Name
    In the "Outgoing mail server (SMTP)" field, verify the server name matches your provider's documentation exactly. Common examples: smtp.gmail.com (Gmail), smtp-mail.outlook.com (Outlook.com), smtp.mail.yahoo.com (Yahoo). Even a single typo will cause authentication to fail.
  3. Check Incoming Server Name
    Verify your incoming server (IMAP or POP) name is also correct. For Gmail it's imap.gmail.com, for Outlook.com it's outlook.office365.com.
  4. Test Again
    Click OK and send a test email. If error 0x8004210B persists, move to the antivirus check below.
Typos in server names are surprisingly common. If you find and fix one, the error usually clears immediately.
4

Disable Antivirus Email Scanning Temporarily Easy

  1. Open Your Antivirus Software
    Find your third-party antivirus in the system tray (bottom right of your taskbar) or open it from your Start menu. Windows Defender rarely causes this issue, so focus on Norton, McAfee, Kaspersky, Bitdefender, or similar products.
  2. Look for Email Protection or SMTP Scanning
    In settings or preferences, search for "email protection," "mail scanning," or "SMTP scanning." This feature monitors outgoing emails. Disable it temporarily (not permanently, we'll fix this properly in a moment).
  3. Restart Outlook
    Close Outlook completely, then open it again.
  4. Send a Test Email
    Try to send an email. If error 0x8004210B disappears, your antivirus was the culprit.
  5. Re-enable Antivirus Protection
    Go back into your antivirus and turn email protection back on. Then add Outlook or your email client to the exceptions list so future emails aren't blocked.
If email sends successfully after disabling antivirus email scanning, this was your problem. Configuring exceptions is faster than disabling protection every time.

Advanced Fixes for Persistent Outlook error 0x8004210B SMTP Issues

5

Remove and Re-add Your Email Account Medium

  1. Open Account Settings
    In Outlook, go to File > Account Settings > Account Settings.
  2. Select and Remove Your Account
    Click the problematic email account to select it, then click the Remove button. You'll get a warning, confirm that you want to remove it. This does not delete your emails; they stay on the server and in your local data files.
  3. Restart Outlook
    Close Outlook completely and reopen it.
  4. Add the Account Back
    Go to File > Add Account. Choose "Manual setup or additional server types" if available, or let Outlook auto-detect first. If you choose manual setup, enter your email address, then select IMAP or POP3 for incoming, and SMTP for outgoing.
  5. Enter Correct Settings
    When prompted, enter the exact server names, port numbers, and encryption types from your provider's documentation. This is your chance to get everything right from the start.
  6. Test Sending
    Once the account is added, send a test email to check if error 0x8004210B is gone.
Removing and re-adding the account clears bad configuration data and forces Outlook to authenticate fresh. This fixes the problem for most users where basic settings are correct.
6

Clear Stored Credentials Medium

  1. Open Credential Manager
    Press Windows key + R, type control /name Microsoft.CredentialManager and press Enter. Or go to Control Panel > User Accounts > Credential Manager.
  2. Find Email-Related Credentials
    Look through the "Windows Credentials" and "Generic Credentials" sections. You're looking for entries that mention your email provider (Gmail, Outlook, Yahoo, etc.) or Outlook itself.
  3. Delete Old Credentials
    Click each credential related to your email and select Remove. This forces Outlook to authenticate again with fresh credentials the next time you send or receive mail.
  4. Restart Outlook
    Close and reopen Outlook. Sign in when prompted.
  5. Test Email Again
    Try sending a message. Outlook should re-authenticate with the server using new credentials.
Stale or corrupted stored credentials can cause authentication errors even if your settings are correct. Clearing them is a quick way to reset authentication.
7

Create a Fresh Outlook Profile Advanced

  1. Open Mail Settings
    Go to Control Panel > Mail (or Mail and Calendar Settings in newer Windows). Click "Show Profiles."
  2. Create New Profile
    Click New and give the profile a descriptive name, like "Outlook Clean" or "MyEmail New". Click OK.
  3. Add Your Email Account
    Outlook will open and ask you to set up an account. Enter your email address and password. If offered manual setup, use it and enter your server details carefully from your provider's documentation.
  4. Configure SMTP Settings Correctly
    Once the account is set up, go to File > Account Settings > Account Settings > Change > More Settings and verify port, encryption, and server names match your provider exactly.
  5. Test Sending
    Send a test email from this new profile.
  6. Switch to the New Profile
    If email sends successfully, you've found that the old profile was corrupted. From now on, start Outlook with this new profile (Control Panel > Mail > Show Profiles, then select it at startup).
A fresh profile is like a clean slate. If your settings are correct but error 0x8004210B persists, profile corruption is likely. The new profile bypasses all old configuration data.
8

Repair Your Outlook Installation Advanced

  1. Open Apps and Features
    Right-click the Start button and select Settings, or go to Control Panel > Programs and Features.
  2. Find Microsoft Office
    Scroll down and find Microsoft Office or Microsoft 365. Click it once.
  3. Run Quick Repair First
    Click Repair (or Change, then Repair). Select Quick Repair and let it run. This takes a few minutes and fixes minor file issues.
  4. Restart Outlook
    Close and reopen Outlook, then test sending an email.
  5. Run Online Repair if Needed
    If error 0x8004210B persists, go back to Office > Repair and select Online Repair this time. This is more thorough and may download components from Microsoft. It can take 10, 15 minutes.
  6. Restart Computer and Test Again
    Reboot your computer after Online Repair completes, then try sending email.
Outlook repair fixes corrupted installation files and runtime libraries that can cause authentication problems. Most users won't need this, but it's the last line before reinstalling.

When to Call Remote Support

If you've worked through all the steps above and error 0x8004210B is still blocking email, your OUTLOOK SMTP settings are now correct but something else on your system is interfering. We can diagnose and fix this remotely in 30 minutes or less. Common culprits at this stage are misconfigured firewalls, VPN conflicts, or deep-seated Outlook profile corruption that resists the standard fixes. Remote support gets you back to sending email without guessing.

Preventing Outlook error 0x8004210B in the Future

Once you've fixed this, don't let it happen again. Start by documenting your email provider's SMTP settings right now. Write down the server name, port, and encryption type in a password manager or secure note. This takes 30 seconds and saves you hours the next time you set up a device or switch email clients.

Keep SMTP authentication enabled in Outlook at all times. It's a security feature, not an inconvenience. If you change your email password or enable multi-factor authentication, update your Outlook account settings immediately. Many users change their password elsewhere but forget to update Outlook, then wonder why they can't send email.

Configure your antivirus to exclude Outlook from email scanning, rather than disabling the antivirus entirely. Most third-party security software lets you add exceptions. This way you get protection without the constant friction.

Keep Windows and Outlook updated. Microsoft releases fixes for authentication and connectivity bugs regularly. A pending Windows update or Office update can sometimes cause SMTP issues that disappear the moment you install it. Check for updates monthly at minimum.

If you use multiple email accounts in Outlook, maintain one clean profile per email environment. Don't layer new accounts on top of old ones repeatedly. When you need a fresh start, create a new profile rather than trying to salvage the old one by removing and re-adding accounts endlessly.

Outlook error 0x8004210B Summary

Error 0x8004210B is SMTP authentication failure, and it's almost always caused by misconfigured settings, disabled authentication, or antivirus interference. Nine times out of ten, checking your port and encryption type against your email provider's documentation fixes it. The remaining cases usually need either a fresh account setup or a new Outlook profile. You now have the tools to solve this yourself. Start with the quick fix, work through the intermediate solutions if needed, and move to the advanced fixes only if basic settings are already correct. If you're stuck after all these steps, remote support is your fastest path to a working inbox.

Frequently Asked Questions

Error 0x8004210B tells you Outlook couldn't authenticate with your SMTP (outgoing mail) server. This happens when your server settings are wrong, authentication is switched off, or security software is blocking the connection.

Many third-party antivirus products include email protection that inspects or blocks SMTP traffic. If this causes problems, disable the email scanning feature in your antivirus settings or add Outlook to the exceptions list.

This depends entirely on your email provider. Port 587 typically uses TLS encryption, whilst port 465 typically uses SSL encryption. Check your provider's support documentation for the correct combination, mixing the wrong port and encryption type together will cause authentication to fail.

Find your email provider's support documentation or account settings page and look for the exact SMTP server name, port number, and encryption type required. Common providers like Gmail, Outlook.com, and Yahoo have specific requirements that must be matched character-for-character.

No. Creating a new profile doesn't delete emails at all. Your emails remain in your data files. A new profile simply creates a fresh configuration. You can switch back to your old profile whenever you want.