Define What Must Move
Start with a source-to-target inventory. The word “email” can hide shared mailboxes, aliases, delegated access, calendars, contacts, distribution groups, archives, mobile devices and applications that send through the old service. Record the owner and acceptance test for each item.
| Discovery question | Why it changes the plan |
|---|---|
| Which mailboxes and aliases are active? | Prevents abandoned accounts from being moved and required addresses from being missed. |
| What data types must transfer? | Email-only tools may not preserve calendars, contacts, permissions or rules. |
| Which systems send as the domain? | Websites, scanners, CRMs and billing tools may need new credentials or connectors. |
| How much downtime is acceptable? | Determines whether a staged, batch or direct cutover is appropriate. |
| What must remain at the source? | Defines coexistence, retention and rollback requirements. |
The Migration Runbook
Write the sequence before the maintenance window. Assign owners and a pass/fail test to every stage.
- Record the current environment. Export users, aliases, groups, forwarding, DNS, applications, retention settings and mailbox sizes.
- Prepare and license the destination. Create identities, verify domains and configure policies without interrupting current mail flow.
- Choose the supported migration path. Confirm exactly which data the selected tool copies and what must be rebuilt manually.
- Reduce DNS uncertainty. Review TTLs early and document the current MX, SPF, DKIM and DMARC records.
- Run a pilot. Test representative mailbox sizes, folders, calendars, delegates and client devices.
- Pre-stage data. Copy historic content while the source is still live, then review errors and exceptions.
- Cut over mail flow. Change only the planned records, preserve the old configuration and test external-to-internal and internal-to-external mail.
- Complete the delta and handover. Copy final changes, reconfigure applications and devices, confirm authentication, support users and retain the source until acceptance.
What to Verify After Cutover
Test real messages rather than relying only on green dashboard indicators. Confirm new inbound and outbound delivery with external recipients, inspect headers for SPF and DKIM, verify DMARC alignment, test replies and aliases, and check applications that send automatically.
Keep a defined rollback and recovery period. Migration tools copy different data types and some data received during a migration may require a final pass.
Common Migration Mistakes
- Changing MX before destination users and authentication are ready.
- Assuming calendars, contacts, delegates and rules are included because email is included.
- Replacing SPF instead of merging every legitimate sender.
- Forgetting website forms, scanners, CRM and transactional applications.
- Running the whole company as the first test batch.
- Closing the source before errors, deltas and user acceptance are complete.
Sources and Further Reading
Platform requirements and recommended practices change. These primary sources were checked when this guide was reviewed.
- Google Workspace data migration service — primary documentation consulted for this guide.
- Microsoft Exchange Online migration batches — primary documentation consulted for this guide.