This will enable you to send emails out with your custom email address which you have set up in Email Forwarding.
For Outlook Express go to Tools > Accounts
Select your current email account from the list and choose "properties".
Now in the 'General' tab simply change your email address to your address that you have chosen in Email Forwarding.
If you get an error when sending you will need to change the address back to what it was before the change and enter your new address in the 'reply to' field in the 'general tab' in that same window.