outlook 2016 wont connect to exchange 2016

we have just re built or exchange 2016 server which is running on Windows 2016 server standard in a VM.

We have configured our internal and external url's and configured all of our users etc.

I can access the mailboxes using owa internally and externally, but when we try to connect using outlook 2016, we get part way through the setup and we are asked for the user name and password. nothing we enter here seems to work. I have tried the automatic and manual connections but we just cant get outlook to connect locally.

Can anyone help in trying to resolve this extremely frustrating problem?

Many thanks.