I am looking for some help with Lync. I am currently running Lync Server 2010. We are in the process of migrating domains, so we currently have a two way trust between the domains. The user account have been merged (test.user1 = tuser1 on the new domain). Our parent company doesnt have a Lync Server setup yet, and they dont want to migrate our legacy Lync Server over to the new domain. In our test environment, I tried migrating a workstation and attempted to connect to the Lync server (now on a different domain, but a 2-way trusted domain) and am receiving a certificate error. Is there a way to allow a pass through so that users can use/connect to the legacy lync server while on the new domain until the parent company sets up Lync on the new domain? Any help is appreciated.

Try the following:

1. Confirm the certificate has been imported to the client machine Trusted Root Certification Authorities 
2. Delete the EndpointConfiguration.CACHE and restart the Lync client
Win7, Vista: LOCALAPPDATA%\Microsoft\Communicator\
Windows XP: %USERPROFILE%\Local Settings\Application Data\Microsoft\Communicator 
3. Try to sign in manually
Lync client options -> Personal -> My Account -> Advanced -> Select manual and enter the FQDN of the FE server (If you already had it set to manual make sure you are using a FQDN).  

If that doesn't resolved your issue please give more details on the certificate error you are getting.

Hi,
If the domains are not in a single forest. A Lync Central Forest deployment is required.
Since the Lync is a forest level server, you have to use FIM, you have to use Microsoft Forefront Identity Manager 2010, Microsoft Identity Lifecycle Manager 2007 Feature Pack 1 to synchronize data across your forests.
For details:
http://technet.microsoft.com/en-us/library/gg670909(v=ocs.14).aspx

I think the certificate will be needed from the CA on the new domain. I was thinking that the Personal>Certificate from the Commincations Server issued to my SIP address would have been sufficient. I will import a certificate and let you know if that works. I appreciate the help.

Hi,

Is there any update on the issue?