EdbMails is a multi-threaded programme that guarantees high performance and doesn't put any limits on the size of the mailbox, the number of items, or the speed of the migration. It can also handle Office 365 throttling and automatically reconnect after a set amount of time, making sure that a PST to Office 365 migration goes well. https://www.edbmails.com/pages/pst-migration-tool.html #PSTtoOffice365