In a single domain/forest that coexistence and migration seems straight forward. We are planning on creating trusts between 2 domains / forests and migrate mailboxes from one domain to the other, from 2010 to 2013.
Is it still possible to have the 2013 servers proxy the connection to 2010 even though it will be across domains? The domains/forests have 2 way trusts built.
The goal is to be able to make all the ip / dns changes so that the traffic starts flowing through 2013 and we will be able to move mailboxes without the end users feeling the pain of reconfiguring all devices.