Hi @navid Talesh ,
Actually NO, you still have to run the cmdlets to prepare Schema/AD/ADdomain for the second domain, even if you are not going to install Exchange on it and only to create mailboxes for the AD users.
Based on my test, before I run
<ExDrive>:\Setup.exe /IAcceptExchangeServerLicenseTerms /PrepareSchema
<ExDrive>:\Setup.exe /IAcceptExchangeServerLicenseTerms /PrepareAD
<ExDrive>:\Setup.exe /IAcceptExchangeServerLicenseTerms /PrepareDomain
I met the same error ( domain controller wasn't found in domain) when I tried to create mailbox with a existed user in child domain.
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