Outlook 2007will not send emails

Hi I have been having problems connecting to the internet and that now seems to be ok, when I try to send an email or reply to an email I get this error message:
550 RCPT TO> Relaying not allowed - please use SMTP AUTH

I have spoken to |Tiscali, and I can connect on their site to send and receive, any help would be great. Thanks

Thanks, I have managed to fix the problem, it had to do with the set up of Outllook in respect of ticking extra boxes when setting up the emails.

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