If an email is stuck in the Outlook Outbox and it's affecting only one user, there could be several reasons for this issue. Here are some common causes and potential solutions:

1. Large Attachments: The email may have large attachments that are taking a long time to upload. Ask the user to check the size of the attachments and consider sending them separately or using a file-sharing service.

2. Outgoing Mail Server Issue: Ensure that the user's outgoing mail server settings are correct. Verify the SMTP server address, port number, and authentication credentials.

3. Internet Connectivity: If there's a problem with the user's internet connection, the email might not be able to send. Have the user check their internet connectivity.

4. Antivirus or Firewall Interference: Antivirus or firewall settings on the user's computer could be blocking the email from being sent. Temporarily disable antivirus or firewall software and try sending the email again.

5. Corrupt Outlook Profile: The user's Outlook profile might be corrupted. Create a new Outlook profile for the user and see if the issue persists.

6. Add-Ins Conflict: A third-party add-in may be causing conflicts in Outlook. Ask the user to disable any unnecessary add-ins and check if the email sends successfully.

7. Cached Exchange Mode: If the user is using Cached Exchange Mode, try disabling it temporarily and check if the email is sent.

8. Server Issues: Check if there are any server issues with the user's email provider. Verify with the IT team if there are any problems on the server-side.

9. Update Outlook: Ensure that the user's Outlook is up to date with the latest updates and patches. Outdated software can sometimes cause issues.

10. Recreate Email Profile: If all else fails, consider recreating the user's email profile from scratch.

It's essential to troubleshoot step by step to identify the exact cause of the problem. If the issue persists, it may be necessary to involve the IT support team or Microsoft support for further assistance.