This is apparently a Linux server error message on receiving an attempt to send email through their outgoing system without providing a valid domain - i.e. tiscali.co.uk. This page explains the process (I don't pretend to understand it):
http://www.carbonstudios.co.uk/site/44/180.html
but the purpose is to prevent spammers sending via a system they don't subscribe to.
So probably what is happening is that your computer, on sending mail, is failing to identify itself as being part of tiscali.co.uk - basically Tiscali thinks you are trying to send while connected through another ISP (few ISPs will allow this).
I assume that no-one is trying to send mail via Tiscali while connected through another ISP? Given that, check in Mail Preferences>Accounts that your email address ends with tiscali.co.uk, that the POP server is pop.tiscali.co.uk and the SMTP server is smtp.tiscali.co.uk
If all this is correct, and Tiscali refuses to help (and this isn't necessarily a Mac-specific problem), quite honestly I should think about changing to an ISP that does support Macs.
As an emergency measure I suggest setting up a Yahoo or other free account so that you can at least send mail from their web-based system.