Can't send emails using Orange smtp server

I have set up my iMac to use my Orange email account POP3 and SMTP servers and although I can receive the emails none will be sent. I get the message telling me to check the SMTP server settings, which I have done and also used the connection doctor. I know the settings are correct as I have used the same details on my iTouch, that is connected through the same wireless router as the iMac, and this both receives and sends the emails using the Orange servers. I have the latest version of Snow Leopard with all up-dates, Mac OS X 10.6.4. Any solutions?