There are three common ways to move mail. They are not competitors: each fits a different situation.
IMAP copy (server to server)
Both mailboxes are online and reachable over IMAP. Messages are read from the old server and written to the new one directly.
- Best for: almost every provider change, from one account to hundreds.
- Keeps: every folder, read/flagged/answered state and the original received date.
- Advantages: nothing to download or upload on your computer; it can resume, run in parallel and keep syncing new mail until the MX switch.
- Limits: both servers must allow IMAP, and provider upload limits apply.
PST (Outlook data file)
A PST file is what Outlook exports. You meet it when the old mail only exists in someone's Outlook: POP3 accounts that downloaded everything, archives, or a server that no longer exists.
- Best for: mail that is no longer on any server.
- How: upload the .pst in the file tool and pick the target mailbox; folders are recreated and each message is appended.
- Watch out: PST files are big. Export per person, and delete the file afterwards; it contains all their mail.
MBOX
MBOX is the plain format used by Thunderbird, Apple Mail and Google Takeout.
- Best for: Takeout exports, Thunderbird profiles, and downloading a mailbox before closing an account.
- Takeout tip: Gmail labels are stored inside the file; the importer can turn them back into folders, so Inbox, Sent and your own labels land in the right places.
- Export: the file tool can also download any IMAP mailbox as a ZIP of MBOX files, one per folder: a simple archive before an account is cancelled.
Quick decision
| Situation | Use |
|---|---|
| Both old and new mailboxes are online | IMAP copy |
| Old mail only in Outlook on a PC | PST import |
| Google Takeout or Thunderbird export | MBOX import |
| Keeping an archive before closing an account | MBOX export |
| Messages too large for the new server | MBOX export for those, IMAP copy for the rest |