It's nonsense that multiple BT email addresses wouldn't work with Microsoft Outlook.
The people on the BT helpline simply meant that they would only help you/explain how to do it if you were using Microsoft Outlook Express.
There are so many different email programs available that you can't expect a service provider's helpdesk to give an explanation for each and every one of them. Mind you, Microsoft Outlook is extremely common, so they could provide help with that.
It's a bit like a garage/petrol station who will happily sell petrol to anyone, but will only provide advoice on fuel consumption if you have a Fiat. There's nothing wrong with using their petrol in your Volkswagen, it's just that they can't give you any information on its fuel system.