Migrating email to Microsoft 365: what to prepare before the move

Define the source and destination

Migrating from an external server to Microsoft 365 and migrating between two tenants are different scenarios. Start by identifying the source system, mailbox count, data volume, domains, and dependencies. Choose the method based on that assessment.

IMAP migration transfers messages from mail folders; contacts, calendars, and tasks need to be handled separately. Destination mailboxes must be prepared before migration. Moving email is not the same as migrating the entire work environment.

When the move is between tenants

Native cross-tenant mailbox migration requires configuration in both environments and has specific identity and licensing requirements. Microsoft documents a per-user migration license for this scenario. Confirm the current requirements before finalizing the budget.

The project must also address domains, addresses, permissions, and applications. OneDrive, SharePoint, and Teams need their own planning: moving a mailbox does not move all of these services.

Prepare a pilot and the transition

1. Inventory: record mailboxes, aliases, groups, shared calendars, and devices that send email.
2. Pilot: test a representative group and document failures and timings.
3. Communication: explain when the change will happen and how to get help.
4. Validation: confirm sending, receiving, access to historical messages, and email client configuration.
5. Continuity: define acceptance criteria and a plan for handling issues.

Avoid promising a disruption-free transition before assessing the environment. The goal is to anticipate dependencies and reduce the impact on the team.