Migration from Exchange 2007 to 2013 between different domain

Hi,

Our company have two domains controller (AD1 and AD2) but it is not trusted. Due to company change, we need to plan merge two domains to new domain (AD3). Under the plan, we will user ADMT for user object migration and password.

For Exchange, we will plan to use Exchange 2013 on new domain. I want to know that what is the best way to migrate all mailboxes from AD1 and AD2 to AD3's new exchange server. We need to migrate over 500+ mailboxes.

Please anyone to suggest what is the best way to do it ?

thanks