Shoviv exchange migrator can process unlimited mailboxes from the on-premises exchange/office365 to on-premises exchange/office365. This software work beyond the size restriction and allow user to process even large mailboxes. The originality of the mailbox folder and the integrity of the emails will be restored as they were before the migration. https://appsource.microsoft.com/en-us/product/web-apps/shovivsoftwarepvtltd1648288209598.exchangeservermigration?tab=Overview