Email migrations fail on timing, not on technology. Plan the switch for a quiet afternoon and keep the old mailbox running for a week.
The order that works
- Create every user before touching DNS
- Migrate old mail with the Data Migration tool
- Change MX records last, with a low TTL
- Add SPF, DKIM and DMARC the same day
On day one, check shared calendars, group aliases and phone setup. That is where the support tickets come from.
