How to Fix Microsoft Outlook 'Invalid Recipient' or '550 5.1.1 User Unknown' Errors When Sending Emails

Summary

This guide helps you troubleshoot and resolve: How to Fix Microsoft Outlook 'Invalid Recipient' or '550 5.1.1 User Unknown' Errors When Sending Emails. Follow the steps below to fix the issue.

Common Causes

If you receive an "Invalid Recipient" or "550 5.1.1 User Unknown" bounce-back message when sending an email through Microsoft Outlook, it means the receiving mail server cannot find a mailbox for the address you entered. This is usually caused by a typo in the email address, an outdated cached address, or an issue with the recipient's domain. This guide provides steps to resolve these errors for Microsoft 365 users.

Quick Fix Steps

1. Double-check the recipient's email address for spelling errors.
2. Delete the recipient from your Outlook Auto-Complete list and re-enter the address manually.
3. Send a test email to a different address to confirm your account is sending correctly.
4. Ask the recipient to confirm their email address is active and spelled correctly.

Detailed Instructions

Clearing the Outlook Auto-Complete Cache

Outlook often stores email addresses in a local cache (Auto-Complete). If a recipient has changed their email address, Outlook may continue trying to send to the old, invalid one.

1. Open Microsoft Outlook.
2. Start a new email and begin typing the recipient's address in the To field.
3. When the incorrect address appears in the drop-down suggestion list, hover over it.
4. Click the X icon next to the address or press Delete on your keyboard to remove it.
5. Type the full email address manually and send your email again.

Verifying the Recipient Address

Ensure there are no hidden spaces or typos in the email address. Common mistakes include:

• Missing the @ symbol or the domain extension (e.g., .com.au).
• Extra spaces at the beginning or end of the address.
• Using a legacy domain that the organisation no longer uses.

Checking Your Own Account Settings

If you are getting these errors for every email you send, regardless of the recipient, there may be an issue with your Outlook configuration.

1. Open Outlook and go to File > Account Settings > Account Settings.
2. Select your Microsoft 365 email account and click Change.
3. Ensure the Email Address field matches your actual mailbox address.
4. Click Next and follow the prompts to finish, then restart Outlook.

Troubleshooting

If the error persists for one specific recipient

The issue is likely on the recipient's end. Their mailbox may be deleted, suspended, or their domain's DNS (MX records) may be misconfigured. You should contact the recipient via phone or another method and ask them to check with their own IT provider.

If the error occurs for all recipients

If every email you send bounces with a 5.1.1 error, there may be a problem with your Microsoft 365 license or mailbox configuration.

1. Log into the Microsoft 365 portal to ensure your license is active.
2. Try sending an email via Outlook on the Web (OWA) at outlook.office.com.
3. If emails send successfully via the web but not the desktop app, your Outlook profile may be corrupted.

If you have followed these steps and are still experiencing bounce-back errors, please Open a Ticket with our support team.

Related Articles:

• Setting Up an Email Account in Outlook (POP/IMAP/Exchange)
• Managing Email Signatures in Outlook
• Troubleshooting Outlook Send/Receive Errors
• Backing Up and Restoring Outlook Data Files (.pst/.ost)
• Organizing Emails with Rules and Alerts in Outlook
• Using Outlook Calendar and Scheduling Meetings

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