A mailbox migration copies everything in a person's mailbox to their target account — folders, mail, calendars, contacts and settings — then keeps it current with delta passes until you cut over. The source stays live throughout.
What moves with a mailbox
| Item | How |
|---|---|
| Folders | The whole tree, nested folders included; archive mailboxes too. Deleted Items and Junk are skipped. |
| Messages | With the original sent/received dates, read state, flags, categories and importance. Drafts stay drafts. |
| Calendars | Events with recurrence, exceptions, reminders and free/busy. Attendees are kept as text in the body so nobody gets re-invited. |
| Contacts | Contact folders and contact groups. |
| Settings | Signature, out-of-office and mailbox rules where the platform allows; otherwise listed as a note. |
Running a pass
- Start pilot usersOn Users, tick a few and press Migrate selected. Watch Migration; open a mailbox in the target and check folders, dates and read state.
- Migrate the rest in wavesBy department, by site, or biggest mailboxes first. Passes are resumable and never copy the same item twice.
- Run delta passesMigration → Run delta pass whenever you like. Only new and changed items move. They are free.
- VerifyBefore cutover, Verify everyone compares every folder with the source. See Verification and retries.
Inside a pass
A pass is a chain of short jobs. Each lists one page of a folder from a saved cursor, skips what it has already copied, copies the rest in chunks through memory, records source ID → target ID, then hands the cursor on. Microsoft's and Google's rate limits are honoured across every worker: when the platform says slow down, everything slows down, and nothing is lost.
Timing
- Most mailboxes: minutes to an hour or two.
- Very large mailboxes (50 GB and up): Microsoft limits how fast a single mailbox can be read, so a day or more. Start them first; delta passes afterwards are quick.
- Everything runs in the background. Close the tab; notifications tell you when a pass finishes.
Messages larger than the target's limit(typically 150 MB on Microsoft 365, 50 MB on Gmail) can't be delivered and are listed by name on the Migration tab.