Over the past few years, I've migrated many mailboxes into Exchange Online. I've talked to customers with every possible set of requirements, and I've been able to accommodate most of them. Until recently, there has been one category of migration requirements that I haven't been able to accommodate: multi-forest migrations. For customers with two or more separate Active Directory (AD) forests, each with their own Exchange deployment, the supported solutions for moving to a single Office 365 tenant were very limited. They could either consolidate their on-premises AD forests into a single forest, or they could contact Microsoft Consulting Services (MCS) and develop a custom solution using Forefront Identity Manager.