We have a customer who is in the process of migrating from one domain ("domain 1") to another so the domain name that will match their current company name ("domain 2"). They have moved a majority of their client PCs from domain 1 to domain 2. The Exchange servers are still in domain 1 and using credentials for domain 1.
After moving to domain 2, users started reporting intermittent Outlook connectivity issues and that they were unable to search in Outlook. Domain 1 has an internal primary DNS zone for their public email domain. In testing, we found that Outlook functioned properly when not connected to the internal network and for client PCs still on domain 1. I originally added conditional forwarders for the public email domains to forward queries from domain 2 to the internal zone on domain 1. This resolved the connectivity issues, but their Outlook search still did not work. I removed the conditional forwarders and duplicated the internal primary DNS zones on domain 2, which resolved the issues.