I am not much familiar with dell but, in my earlier days doing the same job to migrate users mailboxes from Exchange to Office 365, I tried LepideMigrator for Exchange (www.lepide.com/.../office-365-migration.html ) and it was an awesome experience with them.
The plus point of using Lepide tool is that, you can accomplish the job without any downtime in minimum duration.
You can obtain more information about their services from here : www.lepide.com/exchange-migration-services.html