Email Delivery Failure – 5.1.1 Unknown To Address

Hi @Moses Jeremiah Mundo Cosa

Thanks for sharing your experience. I understand how frustrating it is when you're trying to send an email and get an error message. 

The "5.1.1 – Unknown To address" error is a non-delivery report that means the email server couldn't find the specific mailbox you were trying to reach. This is an issue on the recipient's side, but it's something you can often fix by double-checking a few details. 

You can try a few things to resolve this: 

• Check the address: Try removing the saved address from your "To" field and retype it carefully. A small typo can often cause this issue. 
• Verify with the recipient: If possible, check with the person you're trying to email to confirm the exact spelling of their address. 
• Contact their support team: Reaching out to their IT or help desk can ensure the mailbox is active and set up to receive your messages. 

If you’ve already tried these steps and the error keeps appearing, it would help us to have just a little more context so we can guide you better. Whenever you’re comfortable, could you share: 

• The full content of the Non-Delivery Report (NDR) you received. This bounce-back message usually contains important error codes and server details. Please copy the entire message, but kindly remove any personal or sensitive information. 
• The email app or service you used (for example, Outlook on the web, Outlook desktop, mobile app, etc.). 
• If you’ve managed to email this address successfully before, and what type of account you’re sending from (personal, work, or school). 

Just a few details like these can go a long way in helping us understand what might be going on. 

We’ll do our best to help you find the root cause and get this sorted out. Please feel free to share that info when you’re ready.

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