Outlook & VPN

Hello everyone,
Since we switched to a new domain, users are not able to use outlook. First, it was saying that Outlook cannot find server- we changed the confugurations-including DNS and TCP/IP to make sure all is pointing to the new server. They can map to internal folders and drives but they have not been able to use Outlook.
Also, we tried outlook web access and it works-
Not sure why they can`t use the regualr outlook.
Please Help
Thank you