Outlook error 0x8004210B

Question

My outlook Sending Reported Error – 0x8004210B

How can I fix that

Answer 1

Hi @Rosie Brunson

Thank you for posting your question in Microsoft Q&A. 

We understand that you are receiving emails error via Outlook. The error code 0x8004210B generally indicates a timeout while waiting for a response from the outgoing (SMTP) server.  

You could follow these steps to troubleshoot it: 

Change the SMTP Port and Encryption 

Many external networks block default email ports.  

In Outlook: File > Account Settings > Account Settings > [Your Account] > Change > More Settings > Advanced 

Try using port 587 with TLS encryption instead of 25 or 465. This port is less likely to be blocked on public Wi-Fi. 

Allow Outlook Through Windows Firewall 

Sometimes the firewall doesn’t recognize that Outlook should have full internet access.  

Open Control Panel > Windows Defender Firewall 

Click “Allow an app or feature…” 

Make sure Outlook.exe is checked for both Private and Public networks. 

Try a Mobile Hotspot 

Use your phone as a hotspot. If Outlook sends emails this way, it confirms the current Wi-Fi is blocking outgoing connections.  

Increase Outlook’s Server Timeout 

Especially useful on slow or restrictive networks.  

In Account Settings > More Settings > Advanced, increase the Server Timeout slider to 2 minutes. 

Use a VPN 

A VPN can bypass network-level restrictions. If your company provides a VPN, try enabling it before sending emails.  

Please understand that our initial reply may not always immediately resolve the issue. However, with your help and more detailed information, we can work together to find a solution. 

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Answer 2

Aside from the port, another common cause can be the wrong encryption type. A common "slip" when encryption is required that during the initial configuration the wrong encryption method is selected.