Users from one domain can’t access OWA?

The WAN consists of 3 networks and 3 domains. One network consists of Exchange Server and Proxy server in one domain, which I will call Dom1. All users in Dom1 can access OWA through the internet.
The second network which is domaain Dom2 has users that can’t access OWA through the internet.
This all occurred when I implemented a proxy server which is in domain Dom3. In this domain I have created an administrator user and demoted the actual administrator account. I don’t have any trusts from Dom3 to Dom1 or Dom2. I tried and I can’t see the other domains. Can anyone help in this situation?