Below are suggested troubleshooting steps to fix the issue:

1. Verify your Outlook is up-to-date, including any updates to your anti-virus protection.
2. Confirm that you have you signed in with the correct account.
3. Check that you have not switched default sending accounts.
4. Review your security settings to make sure they are allowing Outlook to send emails.
5. Check that you are sending emails from the correct email address.
6. Verify your SMTP settings are correct and configured properly.
7. Make sure you have configured any outbound server authentications.

If none of the above resolutions work, please contact your local IT administrator for further assistance.