Outlook Error 0x80040900: How to Fix Mail Delivery Failures

You see error 0x80040900 when Outlook cannot send or receive emails. This error indicates a problem with your mail server connection or account settings. The message often appears with text about mail delivery failure. This article explains the common causes and provides steps to resolve the issue.

Why Outlook Shows Error 0x80040900

Error 0x80040900 is a generic connection failure code. It means Outlook cannot communicate properly with your email server. This can happen for several technical reasons.

A corrupted Outlook data file is a frequent cause. The file where your emails are stored locally may have errors. When this file is damaged, Outlook cannot sync new messages with the server, triggering the error.

Incorrect account settings are another common source. If your incoming or outgoing server names, port numbers, or encryption methods are wrong, the connection will fail. This often occurs after a password change or a server migration by your email provider.

How Add-ins and Antivirus Software Cause the Error

Third-party add-ins can interfere with Outlook’s normal operation. An add-in designed for security or productivity might block network traffic. Antivirus software that scans email traffic can also mistakenly disrupt the connection to your mail server.

Steps to Resolve Error 0x80040900

Follow these methods in order. Start with the quickest repair tool before moving to more involved solutions.

Method 1: Use the Automatic Account Repair Tool

1. Open Account Settings
   In Outlook, go to File > Account Settings > Account Settings.
2. Select Your Account
   Click on your email account in the list to highlight it.
3. Start the Repair
   Click the Repair button above the account list. Follow the on-screen prompts, which may ask for your password.
4. Restart Outlook
   Close and restart Outlook to see if the error is resolved.

Method 2: Recreate Your Outlook Profile

1. Open the Mail Control Panel
   Close Outlook. Press Windows key + R, type control panel, and press Enter. Search for “Mail” and open the Mail (Microsoft Outlook) control panel.
2. Access Profile Settings
   Click the Show Profiles button.
3. Remove the Old Profile
   Select your current profile from the list and click Remove. Confirm the deletion. Do not delete the associated data files when prompted if you want to keep old emails.
4. Create a New Profile
   Click the Add button. Enter a name for the new profile, then enter your email account details as if setting up Outlook for the first time.
5. Set as Default and Test
   Set the new profile to start automatically. Open Outlook with the new profile and try sending a test email.

Method 3: Start Outlook in Safe Mode

1. Close Outlook
   Ensure Outlook is not running. Check the system tray for its icon.
2. Launch the Run Dialog
   Press Windows key + R on your keyboard.
3. Enter the Safe Mode Command
   Type outlook.exe /safe and press Enter.
4. Test Email Function
   Try to send and receive emails. If it works in Safe Mode, an add-in is likely causing the error.
5. Disable Add-ins
   Go to File > Options > Add-ins. At the bottom, select COM Add-ins from the Manage dropdown and click Go. Uncheck all add-ins, restart Outlook normally, and re-enable them one by one to find the culprit.

If the Main Fixes Do Not Work

Outlook Still Shows 0x80040900 After Profile Rebuild

Check your internet connection and firewall. Temporarily disable Windows Defender Firewall or any third-party firewall to test. Verify your email server settings with your IT department or email provider. The server address or port number may have changed.

Error Occurs Only When Sending to Specific Recipients

This points to a problem with the recipient’s server, not yours. Their mailbox may be full, or their server may be blocking your message. Try sending the email to a different address to confirm your sending function works.

Error Appears Intermittently

An unstable network can cause this. Switch from Wi-Fi to a wired connection if possible. For Microsoft 365 accounts, try switching your account connection type. Go to File > Account Settings, select your account, click Change, and toggle between Cached Exchange Mode and Online Mode.

Account Repair vs. Profile Rebuild: Key Differences

Item	Automatic Account Repair	Manual Profile Rebuild
Speed	Fast, automated process	Slower, manual steps required
Data Risk	Very low, does not delete mail	Low, but requires careful backup of PST/OST files
Best For	Minor configuration errors or password updates	Severe corruption of the profile or persistent errors
Outcome	Refreshes server settings and authentication	Creates a brand new, clean profile structure

You can now diagnose and fix the 0x80040900 mail delivery error. Use the built-in repair tool first for a quick resolution. If the problem continues, creating a new Outlook profile is a reliable solution. For advanced users, running the scanpst.exe tool on your Outlook data file can repair deeper file corruption not fixed by other methods.