Dear Nigel Shearing,
Welcome to Microsoft Community.
I'm very sorry that you're having trouble sending emails using the new Outlook, and I'd like to help you.
Could you please explain what exactly is happening when new email addresses don't get acknowledged or accepted?
For example, when you enter a new destination address and click Send, do you get any error messages?
What domain is the e-mail address you add, is it Microsoft domain (Outlook/Hotmail/Live/Msn) or non-Microsoft domain account (gmail.com, yahoo.com etc.)?
Since we are a public forum using text communication, we may not be able to find the crux of the problem immediately. We need you to provide some information and perform some tests.
My first reply doesn't always solve your problem, but your timely feedback will help us better locate your problem.
Looking forward to your timely response, I will continue to follow up on your question here.
Best Wishes,
Stan.L - MSFT | Microsoft Community Support Specialist