HELP - can't send mail with NTL broadband.

Hi,

We've just signed up with NTL broadband. We run two Mac laptops off an Airport Extreme wireless base station. Everything works a treat (Internet surfing, receiving email) except that we cannot send emails from the Mac Mail program - it says unable to send every time. We've tried switching everything off, we've been through all the settings (smtp.ntlworld.com as the outgoing server) with the Help Desk, we're not running Password authentication etc....yet nothing works. The Help Desk said it must be a Firewall or Anti-Virus filter that is getting in the way but we're running neither and it still won't send. They suggest calling Apple but we don't want to go down that grotesquely expensive road. Especially since this feels like a small thing that can be fixed with the right know-how.

Any suggestions would be VERY welcome....

Thanks!