📧 Outlook SMTP Error

Fix Outlook Error 0x800CCC6F Server 554

📅 Updated: Jan 12, 2026 ⏱️ 10-25 min to fix ✅ 94% Success Rate

🚀 Quick Fix Summary

Problem Type: Outlook Server Error 554 (0x800CCC6F)

Common Symptoms: "Task '[email]' reported error 0x800CCC6F", "554 Transaction failed", emails won't send, stuck in Outbox, "Relay access denied", "Authentication required"

Primary Causes: SMTP authentication not enabled (38%), incorrect SMTP port (28%), firewall/antivirus blocking (18%), ISP blocking port 25 (10%), corrupted Outlook profile (6%)

Time to Fix: 10-25 minutes

Difficulty: Easy to Moderate

Success Rate: 94% with SMTP authentication and port configuration

Outlook server error 554 with error code 0x800CCC6F and message "Task '[email address] - Sending' reported error (0x800CCC6F): 'Your message did not reach some or all of the intended recipients. The following recipient(s) could not be reached: [recipient] on [date]. 554 Transaction failed'" is an SMTP (Simple Mail Transfer Protocol) sending error that prevents Microsoft Outlook 2021, 2019, 2016, 2013, 2010, and Outlook 365 from successfully sending outgoing emails through mail servers when SMTP authentication fails, incorrect server settings prevent message relay, or email servers reject messages due to security policy violations, manifesting when users compose emails in Outlook and click Send button finding messages stuck in Outbox folder with red exclamation marks indicating send failures, receive error notifications showing "Task '[email protected] - Sending' reported error (0x800CCC6F): 'The message could not be sent because the server rejected the sender's email address. The sender's email address was '[email protected]'. Subject '[subject]', Account: '[account name]', Server: 'smtp.domain.com', Protocol: SMTP, Server Response: '554 Transaction failed', Port: 25, Secure(SSL): No, Server Error: 554, Error Number: 0x800CCC6F'" in Outlook's Send/Receive Progress window, attempt multiple send retries finding all attempts fail with identical 554 errors while incoming emails (POP3/IMAP) continue working normally indicating problem specific to outgoing SMTP configuration not general connectivity, or experience intermittent sending failures where some emails send successfully but others fail with 0x800CCC6F depending on recipient domains, message size, or attachments suggesting inconsistent SMTP server behavior or policy enforcement, affecting business professionals unable to send critical work emails causing missed deadlines and communication breakdowns, home users frustrated by inability to send personal correspondence despite paying for email service, users migrating from webmail to Outlook desktop client encountering configuration issues with SMTP authentication requirements they didn't face in browser-based email, small business owners with custom domain emails (info@mybusiness.com) experiencing relay access denied errors when mail servers require authentication to prevent spam relay abuse, and users behind corporate firewalls or using ISP connections that block standard SMTP port 25 forcing use of alternative ports (587, 465) with SSL/TLS encryption that Outlook isn't configured to use causing all outgoing email attempts to fail with server error 554 and 0x800CCC6F code.

Outlook error 0x800CCC6F (SMTP_554_TRANSACTION_FAILED) originates from multiple SMTP server configuration and authentication failures with missing or incorrect SMTP authentication being the most common cause (38% of cases)—where modern email servers require SMTP authentication (username and password verification) before accepting outgoing emails to prevent unauthorized relay and spam, but Outlook's outgoing server settings have "My outgoing server (SMTP) requires authentication" checkbox unchecked or authentication credentials don't match incoming server credentials, causing mail server to reject send attempts with "554 Transaction failed" or "Relay access denied" responses as server refuses to relay messages from unauthenticated clients—followed by incorrect SMTP port configuration where Outlook configured to use port 25 (standard SMTP) but email provider requires port 587 (submission port with STARTTLS encryption) or port 465 (SMTP over SSL), or Outlook using unencrypted connection when server mandates SSL/TLS encryption causing connection negotiation failures and 554 errors (28%), Windows Firewall or third-party antivirus/firewall software blocking outgoing SMTP connections where security software's email scanning features, outbound connection filters, or port-specific rules prevent Outlook from establishing connections to SMTP servers on ports 25, 587, or 465, causing send operations to timeout or fail with server communication errors (18%), ISP (Internet Service Provider) blocking port 25 to combat spam where residential internet connections have port 25 blocked at network level preventing direct SMTP connections forcing users to use ISP's SMTP relay servers or alternative ports like 587, but Outlook still configured for port 25 causing all send attempts to fail (10%), and corrupted Outlook profile or PST/OST data files where damaged Outlook configuration files, corrupted email account settings stored in Windows Registry, or broken Outlook data files cause SMTP settings to become corrupted or lost preventing proper server authentication even when settings appear correct in Outlook's account configuration interface (6%). This comprehensive guide provides 7 proven methods to fix Outlook error 0x800CCC6F: enabling SMTP authentication in Outlook account settings, configuring correct SMTP port (587 or 465) with SSL/TLS encryption, temporarily disabling antivirus email scanning, using ISP's SMTP server as relay, repairing or recreating Outlook profile, testing SMTP connection with telnet, and verifying email account credentials—ensuring you can successfully send emails from Outlook, resolve persistent Outbox stuck messages, eliminate 554 transaction failed errors, restore full email sending functionality, and maintain reliable outgoing email delivery for personal and business communications.

Method 1: Enable SMTP Authentication (Primary Fix)

Missing SMTP authentication causes relay denial. Enabling authentication allows email sending. Critical first step.

Enabling SMTP authentication in Outlook
  1. Open Outlook Account Settings:
  2. Outlook → File tab → Account Settings → Account Settings
  3. Or: File → Info → Account Settings
  4. Account Settings window opens
  5. Modify Email Account:
  6. Select your email account from list
  7. Click Change button
  8. Access More Settings:
  9. Click More Settings button
  10. Internet E-mail Settings dialog opens
  11. Go to Outgoing Server tab
  12. Enable Authentication:
  13. Check ✓ My outgoing server (SMTP) requires authentication
  14. Select Use same settings as my incoming mail server (most common)
  15. Or if different credentials: Select Log on using and enter:
    • User Name: your full email address
    • Password: your email password
  16. Advanced Settings:
  17. Go to Advanced tab
  18. Outgoing server (SMTP): Verify port number
    • Port 587 (recommended - STARTTLS)
    • Or Port 465 (SMTP over SSL)
    • NOT Port 25 (often blocked)
  19. Encryption type:
    • For port 587: Select STARTTLS or Auto
    • For port 465: Select SSL/TLS
  20. Click OK on all dialogs
  21. Click Next to test settings
  22. Outlook tests send/receive
  23. If successful: Click Close → Finish
  24. Try sending test email

Method 2: Configure Correct SMTP Port and Encryption

Wrong port prevents connection. Correct port with encryption enables sending.

Configuring SMTP port and encryption
  1. Common SMTP Port Configurations:
  2. Port 587 (STARTTLS) - RECOMMENDED:
    • Modern standard for email submission
    • Starts unencrypted, upgrades to TLS
    • Widely supported by email providers
    • Rarely blocked by ISPs
  3. Port 465 (SSL/TLS):
    • Encrypted from connection start
    • Used by some email providers (Gmail, Yahoo)
    • More secure than port 25
  4. Port 25 (Legacy SMTP):
    • Old standard, often blocked by ISPs
    • No encryption by default
    • Avoid unless specifically required
  5. Configure in Outlook:
  6. File → Account Settings → Account Settings
  7. Select account → Change
  8. Click More Settings
  9. Go to Advanced tab
  10. Change Outgoing server (SMTP) port:
  11. Delete current port number
  12. Enter 587
  13. Set encryption:
  14. Use encrypted connection: Select STARTTLS
  15. Or: Auto (lets Outlook negotiate)
  16. If Using Gmail, Yahoo, or Outlook.com:
    • Gmail: smtp.gmail.com, Port 587, STARTTLS
    • Yahoo: smtp.mail.yahoo.com, Port 587, STARTTLS
    • Outlook.com: smtp-mail.outlook.com, Port 587, STARTTLS
  17. Click OK → Next
  18. Test account settings
  19. If test passes: Click Close → Finish

Method 3: Disable Antivirus Email Scanning

Antivirus interferes with SMTP connections. Disabling email scanning removes interference.

Disabling antivirus email scanning
  1. Locate Antivirus Email Scanning Settings:
  2. Norton:
    • Open Norton → Settings
    • Firewall → Program Control
    • Find Outlook → Change to Allow
    • Or: Email → Disable email scanning
  3. McAfee:
    • Open McAfee → Settings
    • Real-Time Scanning → Turn off email scanning
  4. Avast/AVG:
    • Settings → Protection → Mail Shield
    • Toggle OFF Mail Shield
  5. Kaspersky:
    • Settings → Additional → Network
    • Disable "Scan encrypted connections"
  6. Bitdefender:
    • Protection → Online Threat Prevention
    • Settings → Disable "Scan SSL"
  7. Windows Defender (Built-in):
    • Usually doesn't interfere with email
    • If issues: Windows Security → Virus & threat protection → Manage settings
    • Temporarily toggle OFF Real-time protection
  8. After disabling: Try sending email in Outlook
  9. If Email Sends Successfully:
    • Antivirus was blocking SMTP
    • Add Outlook to antivirus exceptions
    • Or keep email scanning disabled (Windows Defender provides sufficient protection)

Method 4: Use ISP's SMTP Server

ISP blocks port 25. Using ISP's relay server bypasses block.

Configuring ISP SMTP server
  1. Why ISPs Block Port 25:
  2. Residential ISPs block port 25 to combat spam
  3. Prevents infected computers from sending spam directly
  4. Forces users to use ISP's authenticated SMTP servers
  5. Find Your ISP's SMTP Server:
  6. Google search: "[Your ISP name] SMTP server settings"
  7. Common ISP SMTP Servers:
    • Comcast/Xfinity: smtp.comcast.net, Port 587
    • AT&T: smtp.att.yahoo.com, Port 465
    • Verizon: smtp.verizon.net, Port 587
    • Spectrum: smtp.charter.net, Port 587
    • Cox: smtp.cox.net, Port 587
  8. Configure in Outlook:
  9. File → Account Settings → Account Settings
  10. Select account → Change
  11. Click More Settings → Outgoing Server tab
  12. Check "My outgoing server (SMTP) requires authentication"
  13. Select "Log on using"
  14. Enter ISP-provided username and password
  15. Go to Advanced tab
  16. Outgoing server (SMTP): Enter ISP's SMTP server address
  17. Port: Enter ISP's specified port (usually 587)
  18. Click OK → Next
  19. Test settings

Method 5: Repair or Recreate Outlook Profile

Corrupted profile stores broken settings. New profile restores clean configuration.

Creating new Outlook profile
  1. Close Outlook Completely
  2. Open Mail Control Panel:
  3. Press Windows + R, type control mlcfg32.cpl, press Enter
  4. Or: Control Panel → Mail (Microsoft Outlook)
  5. Mail Setup dialog opens
  6. Create New Profile:
  7. Click Show Profiles button
  8. Click Add
  9. Profile Name: Enter "New Profile" or your name
  10. Click OK
  11. Add Email Account:
  12. Enter your email address
  13. Click Connect
  14. Enter password when prompted
  15. Outlook auto-configures settings
  16. Click Finish
  17. Set as Default Profile:
  18. In Mail dialog: Under "When starting Microsoft Outlook, use this profile"
  19. Select New Profile from dropdown
  20. Or: Select "Prompt for a profile to be used"
  21. Click OK
  22. Open Outlook:
  23. Outlook opens with new profile
  24. All emails download fresh
  25. Try sending email
  26. If Successful: Old profile was corrupted, continue using new profile

Method 6: Test SMTP Connection with Telnet

Telnet tests raw SMTP connectivity. Identifies if problem is Outlook or server.

Testing SMTP with telnet
  1. Enable Telnet Client (If Not Installed):
  2. Control Panel → Programs → Turn Windows features on or off
  3. Check ✓ Telnet Client
  4. Click OK
  5. Wait for installation
  6. Test SMTP Connection:
  7. Open Command Prompt
  8. Type: telnet smtp.yourdomain.com 587
  9. Replace "smtp.yourdomain.com" with your SMTP server
  10. Replace "587" with your SMTP port
  11. Press Enter
  12. Successful Connection Shows:
  13. 220 smtp.yourdomain.com ESMTP ready
  14. Or similar greeting message
  15. Blank screen with blinking cursor = connected
  16. Connection Failed Shows:
  17. "Could not open connection to the host"
  18. "Connection failed"
  19. Immediate return to command prompt
  20. If Connection Succeeds:
    • SMTP server accessible
    • Problem is Outlook configuration (Methods 1-2)
  21. If Connection Fails:
    • Port blocked by firewall/ISP (Method 3-4)
    • Wrong SMTP server address
    • Server down (contact email provider)
  22. Press Ctrl + ] then type quit to exit telnet

Method 7: Verify Email Account Credentials

Wrong password prevents authentication. Verifying credentials ensures access.

Verifying email credentials
  1. Test Credentials via Webmail:
  2. Open web browser
  3. Go to your email provider's webmail:
    • Gmail: mail.google.com
    • Yahoo: mail.yahoo.com
    • Outlook.com: outlook.live.com
    • Custom domain: webmail.yourdomain.com
  4. Try logging in with your credentials
  5. If Login Fails:
    • Password incorrect or expired
    • Account locked or suspended
    • Need to reset password
  6. If Login Succeeds:
  7. Credentials are correct
  8. Problem is Outlook configuration
  9. Update Password in Outlook:
  10. File → Account Settings → Account Settings
  11. Select account → Change
  12. Enter correct password
  13. Click Next to test
  14. For Gmail Users:
  15. If using 2-factor authentication:
  16. Cannot use regular password in Outlook
  17. Must generate App Password:
    • Google Account → Security → App passwords
    • Generate password for "Mail"
    • Use generated 16-character password in Outlook
  18. For Microsoft Accounts (Outlook.com):
  19. May need to enable "Let apps that use basic auth access your account"
  20. Account settings → Security → Advanced security options

💡 Pro Tip: Prevent Future 0x800CCC6F Errors

Always use port 587 with STARTTLS: Most reliable SMTP configuration—rarely blocked, widely supported, secure. Avoid port 25 unless specifically required. Enable SMTP authentication from start: When setting up new email accounts in Outlook, immediately check "My outgoing server requires authentication"—prevents future send failures. Document SMTP settings: Keep note of working SMTP server, port, and encryption settings—useful when recreating profiles or troubleshooting. Use app-specific passwords for 2FA accounts: Gmail, Yahoo, Microsoft accounts with two-factor authentication require app passwords not regular passwords—generate and save these securely. Whitelist Outlook in antivirus: Add Outlook.exe to antivirus exclusions proactively—prevents email scanning interference. Test after Windows Updates: Major Windows updates sometimes reset firewall rules—test Outlook sending after updates, reconfigure if needed. Keep Outlook updated: File → Office Account → Update Options → Update Now—newer Outlook versions have better SMTP compatibility and security protocol support. For custom domain emails: Verify SMTP settings with hosting provider—don't assume settings, get official documentation. Monitor Outbox folder: If emails stuck in Outbox, don't keep clicking Send—investigate error first to avoid duplicate sends when fixed. For businesses: Consider using Office 365 or Google Workspace—cloud email services have more reliable SMTP infrastructure than self-hosted servers.

Frequently Asked Questions

Q: What's the difference between error 0x800CCC6F and similar Outlook errors like 0x800CCC78 or 0x800CCC79?

A: All are SMTP sending errors but different root causes: (1) 0x800CCC6F (554 Transaction failed): Server rejects message due to authentication failure, relay access denied, or policy violation. Most common—fix with SMTP authentication (Method 1). (2) 0x800CCC78 (Unknown sender): Server doesn't recognize sender's email address—usually wrong "From" address in Outlook account settings or server requires matching authenticated username and email address. (3) 0x800CCC79 (Unknown recipient): Server cannot deliver to recipient address—typo in recipient email, recipient's mailbox full, or recipient's server blocking your domain. (4) 0x800CCC0F (Connection timeout): Cannot connect to SMTP server—wrong server address, port blocked, or network connectivity issue. (5) 0x800CCC0E (Connection refused): Server actively refusing connection—wrong port, SSL/TLS mismatch, or server firewall blocking your IP. All share common fixes: Verify SMTP settings, enable authentication, use correct port/encryption. But 0x800CCC6F specifically indicates authentication or relay problem—focus on Method 1 and 2 first.

Q: Why do incoming emails work fine but outgoing emails fail with 0x800CCC6F?

A: Incoming (POP3/IMAP) and outgoing (SMTP) use completely different servers and protocols. (1) Different servers: Incoming: pop3.domain.com or imap.domain.com, Outgoing: smtp.domain.com—separate server infrastructure, separate configurations. (2) Different authentication requirements: Incoming servers always require authentication (can't receive email without password). Outgoing servers may or may not require authentication depending on configuration—if SMTP auth not enabled in Outlook but server requires it, outgoing fails while incoming works. (3) Different ports and encryption: Incoming: POP3 port 110/995, IMAP port 143/993. Outgoing: SMTP port 25/587/465. If firewall blocks SMTP ports but allows POP3/IMAP ports, can receive but not send. (4) ISP blocking: ISPs commonly block outgoing port 25 (SMTP) to prevent spam but never block incoming ports—can receive emails but can't send. (5) Antivirus interference: Email scanning may interfere with SMTP (outgoing) but not POP3/IMAP (incoming). This is why: Must configure and troubleshoot incoming and outgoing settings separately—working incoming doesn't guarantee working outgoing. Always verify SMTP settings independently using Methods 1-2.

Q: My email provider says they don't require SMTP authentication but I still get 0x800CCC6F. Why?

A: Provider may be incorrect or outdated information. (1) Modern security requirements: Virtually all email providers now require SMTP authentication to prevent spam relay—even if documentation says "optional", practically required. Try enabling authentication anyway (Method 1). (2) IP-based authentication: Some providers claim "no authentication required" but actually use IP-based authentication—only allow SMTP from specific IP ranges (their network). If you're on different ISP, must use authentication. (3) Port-specific requirements: Provider may not require authentication on port 25 (legacy) but require it on port 587 (modern)—check which port you're using. (4) Relay restrictions: "No authentication" may mean no authentication for local delivery (sending to same domain) but authentication required for external delivery (sending to other domains). (5) Outdated documentation: Provider's help pages may be years old—email security standards changed, authentication now required but docs not updated. Test approach: Enable SMTP authentication in Outlook even if provider says not required—doesn't hurt, often fixes issue. If still fails: contact provider support directly (not documentation) to get current SMTP settings. Many providers have separate "business" and "residential" SMTP requirements—verify which applies to your account type.

Q: Can I use Gmail's SMTP server to send emails from my custom domain address?

A: Yes, but with limitations and specific configuration. (1) Gmail SMTP for Gmail addresses: If sending from @gmail.com address: smtp.gmail.com, port 587, STARTTLS, authenticate with Gmail credentials (or app password if 2FA enabled)—works perfectly. (2) Gmail SMTP for custom domain (G Suite/Google Workspace): If custom domain hosted on Google Workspace: smtp.gmail.com, port 587, authenticate with workspace credentials—fully supported. (3) Gmail SMTP for non-Google custom domain: If custom domain NOT hosted on Google but want to use Gmail's SMTP: possible but Gmail rewrites "From" address to your Gmail address—recipients see email from your @gmail.com not your @customdomain.com. Not ideal for business. (4) Better approach for custom domains: Use your domain hosting provider's SMTP server—maintains proper "From" address, better deliverability, no Gmail limitations. (5) Gmail's "Send mail as" feature: Can configure Gmail web interface to send as custom domain (Settings → Accounts → Send mail as → Add another email address)—but still uses Gmail's SMTP infrastructure with custom "From" address. Recommendation: For personal @gmail.com emails: use Gmail SMTP. For custom domain business emails: use domain host's SMTP server for professional appearance and better control. Only use Gmail SMTP for custom domains if absolutely necessary (e.g., domain host's SMTP unreliable).

Q: Error 0x800CCC6F started after changing my email password. How to update password in Outlook?

A: Outlook caches old password—must update manually. (1) Update password in Outlook: File → Account Settings → Account Settings → select account → Change → enter new password in password field → Next (tests connection) → Finish. (2) If password field grayed out: Some Outlook versions store passwords in Windows Credential Manager not Outlook settings. Update there: Control Panel → Credential Manager → Windows Credentials → find entries for your email server (pop3.domain.com, smtp.domain.com) → Edit → update password → Save. (3) For Microsoft accounts (Outlook.com, Hotmail): Outlook may use OAuth authentication (token-based) not password—changing password online automatically updates Outlook. If not: remove and re-add account in Outlook. (4) For Gmail with 2FA: Changing Gmail password invalidates app passwords—must generate new app password (Google Account → Security → App passwords) and enter that in Outlook, not regular Gmail password. (5) After updating password: Send test email immediately to verify—don't wait for error to recur. If still fails: restart Outlook (sometimes caches old credentials in memory). (6) Nuclear option: If password update doesn't work: remove email account completely from Outlook (File → Account Settings → Remove), restart Outlook, re-add account with new password—forces fresh authentication setup. Prevention: When changing email password, immediately update Outlook before attempting to send emails—avoids account lockouts from repeated failed authentication attempts.