Start with the inventory
List domains, users, aliases, shared mailboxes, distribution lists and applications that send mail. Record approximate data volumes, available access and any source-system failures. Include user devices and mail clients in the discovery.
Validate the destination
Agree storage, identity, certificates, mail flow, backup and administration before migrating users. Check the licensing and features required by the organisation. A platform change may also change how users access their historical data.
Run a meaningful pilot
Choose users who represent different mailbox sizes and workflows. Test mail, calendars, contacts and client access where these are in scope. Record what migrated, what did not and the acceptance criteria for the wider rollout.
Plan the cutover and the fallback
Document DNS changes, sender applications, user communications, support contacts and the point at which the old system stops accepting changes. Set a clear decision point for proceeding, pausing or reverting.
Close with reconciliation
Compare the expected users and data with the migration results. Record exceptions, provide administrator handover and confirm the support arrangement after cutover.
This guide is a planning aid. The scope and design for your environment should be agreed after discovery.