With a few mailboxes you can copy everything and switch the same day. With hundreds, the copy takes longer than a weekend, so the trick is to do the slow part before the weekend and keep the weekend for the switch.
The idea: copy early, sync often, switch late
- Copy all historic mail during the week or two before.
- Keep new mail flowing across with an ongoing sync every few hours.
- On the weekend, switch MX and run one final sync.
The weekend then only moves a few hours of new mail, which takes minutes, not days.
Timeline
| When | What |
|---|---|
| T−14 days | Inventory mailboxes, aliases, shared mailboxes. Create users on the new side. |
| T−10 days | Validate every login. Fix passwords and missing IMAP access. Run the size check. |
| T−10 to T−2 | Full copy in batches of 30–50 mailboxes. Start with the largest; they take longest. |
| T−2 days | Lower MX TTL to 300 seconds. Tell staff what will change and when. |
| Friday evening | Switch MX, SPF, DKIM, DMARC. Leave the ongoing sync running. |
| Saturday | Final sync for every mailbox. Run the source/target comparison. |
| Sunday | Fix what the comparison listed. Prepare device instructions. |
| Monday | People sign in to the new service. Keep the old one read-only for two weeks. |
Batches and parallelism
Providers limit how fast one mailbox can receive mail over IMAP, and how many connections one tenant may open. Running a few accounts in parallel is faster than one at a time, but going too wide only produces "try again later" replies. The console caps parallelism per provider for you; for Microsoft 365, around 2–6 accounts at once is usually the sweet spot.
Let people type their own passwords
Collecting hundreds of passwords is slow and risky. Send invitation links instead: each person opens their link, enters their own password, and their mailbox is copied under your plan. You see the progress, never the passwords.
What to check before you call it done
- The comparison report shows no missing messages, or you know why (for example, messages larger than the new server accepts).
- Aliases and shared mailboxes receive mail on the new service.
- Outgoing mail passes SPF and DKIM (send a test to a Gmail address and view the original).
- Calendars and contacts were moved in their own run if people need them.