• Open Outlook Express
• Click on "Tools" on the top menu
• Choose "Accounts"
• Click the "Add" button on the right and choose "Mail…"
• Type the name you wish to appear on outgoing emails, click "Next"
• Type the full email address your adding
• In the Incoming Mail field type "mail." then your domain name (ex: mail.yourdomain.com)
• In the Outgoing Mail field type type "mail." then your domain name (ex: mail.yourdomain.com)
• Type the full email address in the first field (ex: john@doe.com) and your password in the second. Click "Next"
• Click "Finish"
• Click "Close"
• Now select the email address you have just created from the list and click properties
• Click the "Servers" tab
• At the bottom tick the box for "My server requires authentication"
• Click Settings next to it and ensure it is set to "Use same settings as my incoming mail server"

Your email address should now be setup and working correctly.