I am standing up a new Exchange 2016 environment. Current AD has 3 domains. Domain1, Domain2 and Domain3. We have a sister company that we have established a Domain Trust with and they have 1 domain (Domain4). Domain4 has a pre-existing Exchange 2013 environment and I wish to migrate their mailboxes into our Exchange 2016 environment. Or is the best way to accomplish this to upgrade Domain4's Exchange to 2016? My concern is how account creation would work? I know that we can share GALs but where do the mail-enabled AD accounts need to live? I have not worked with connecting an Exchange server to different forests. Any input to point me in the right direction would be great.