How to Fix Outlook Error 0x8004210a

When sending or receiving email, Outlook may display the following timeout error message: “Sending and receiving reported error (0x8004210a): The operation timed out waiting for a response from the receiving (POP) server.” This prevents mail synchronizations from finishing.

This troubleshooting guide details the causes of 0x8004210a and steps to resolve it on both Windows and macOS by adjusting server timeout thresholds, clearing stuck email attachments, and fixing packet size fragmentation issues.

1. Understand the Cause of Outlook Error 0x8004210a

This error indicates that the Outlook desktop client initiated a connection to the mail server, but the connection timed out before the transfer completed. The primary root causes include:

• Oversized Messages Stuck in Outbox: Large file attachments take longer to transfer than the default 1-minute timeout window configured in Outlook.
• Antivirus Scan Delays: Antivirus email shields intercept incoming data packets, delaying the payload delivery to Outlook and triggering a timeout threshold.
• Packet Fragmentation (MTU Issue): The Maximum Transmission Unit (MTU) size on your local network interface is mismatched, causing large IP packets to fragment and drop on the WAN.
• Slow/Unstable Connection: Remote server throttling or high packet latency on your broadband line.

Step 1: Force Quit Outlook and Clear Stuck Outbox Items

If Outlook hangs when opening because of a massive unsent email, use these commands to close it, switch off the network link, and clear the queue:

:: Terminate Outlook
taskkill /F /IM outlook.exe

:: Launch Outlook in Offline Mode to prevent auto-syncing during startup
start outlook.exe /safe /u

Once open:

1. Go to the Send/Receive tab and click Work Offline.
2. Navigate to your Outbox folder.
3. Right-click the stuck message and select Delete (or drag it to Drafts to edit).
4. Uncheck Work Offline and restart Outlook.

Step 2: Adjust Network MTU Settings

If timeouts occur on normal-sized emails, packet size conflicts are likely. Reset your network’s MTU to standard values (e.g. 1400 or 1450 instead of 1500 to prevent fragmentation):

:: Show active network interfaces (Run as Administrator CMD)
netsh interface ipv4 show subinterfaces

:: Set MTU of your interface (replace "Wi-Fi" with your actual connection name)
netsh interface ipv4 set subinterface "Wi-Fi" mtu=1400 store=persistent

Step 1: Clean Up Stalled Database Transactions

Close all active Microsoft processes and clear local caching directories:

# Force quit Office clients
killall "Microsoft Outlook"

# Clear temporary app cache directories
rm -rf ~/Library/Containers/com.microsoft.Outlook/Data/Library/Caches/*

# Find active network services
networksetup -listallnetworkservices

# Verify current MTU of Wi-Fi (en0)
networksetup -getMTU en0

# Set MTU to 1400 to stop packet loss timeouts
sudo networksetup -setMTU en0 1400

4. Adjust Outlook Server Timeouts Manually

By default, Outlook times out server requests after 1 minute. Increasing this to 3–5 minutes gives the client enough time to process large data payloads:

1. Open Outlook and go to File → Account Settings → Account Settings….
2. Select your POP3/IMAP account and click Change.
3. In the email setup page, click More Settings….
4. Navigate to the Advanced tab.
5. Under Server Timeouts, drag the slider to the right (e.g., 3 minutes or 5 minutes).
6. Click OK, then click Next and Finish.

5. Summary Diagnostics Checklist