For example with Thunderbird, you can create links to two accounts (source and destination) by choosing IMAP access. Then you can move all the emails from one account to another working on a folder at a time.
The process can be very long in terms of time (and boring) especially if your internet connection is slow and you have many mails to be move! In addition, calendars, address book, contacts and documents in Google Drive must be migrated by other processes.
2) use a third-party software
Simply login with domain administrator credentials, and if you must have access to a second domain you must use its administrative account. Then you choose the source account and the destination account, so you begin the process!
[Source Link] Backupify MIGRATOR