Migration discovery
Start with a complete mailbox inventory.
Before choosing a migration method, identify every mailbox, alias, shared mailbox, distribution list, forwarding rule and application that uses email.
The inventory should also record mailbox size, archive requirements, user priority and whether any accounts are inactive or duplicated.
Discovery checklist
- List every active user mailbox.
- Identify shared and departmental mailboxes.
- Record aliases and distribution lists.
- Confirm mailbox sizes and storage use.
- Identify disabled or former-user accounts.
- Record applications that send email.
- Review forwarding and mailbox rules.
- Confirm calendars, contacts and archives.
Discovery is an opportunity to remove obsolete accounts, confirm ownership and reduce avoidable licensing costs.
Licensing preparation
Match each user to the correct Microsoft 365 licence.
Licence selection should reflect mailbox, desktop application, security, device-management and collaboration requirements.
Mailbox Only
Suitable where users primarily require hosted email and basic cloud services.
Desktop Applications
Required where users need supported desktop versions of Outlook, Word, Excel and other Office applications.
Advanced Security
Appropriate where stronger identity, endpoint and information-protection controls are required.
Shared Mailboxes
Shared mailboxes may not need the same licence as ordinary users, depending on storage and access requirements.
User principal names, primary email addresses and licence assignments should be confirmed before the migration begins.
Identity and security
Prepare user access before moving mail.
Migration should not create a period where users have weak passwords, unmanaged access or unclear sign-in instructions.
Confirm User Identities
Match display names, usernames, email addresses and employee records.
Plan Password Handling
Define temporary credentials, first sign-in requirements and password-reset support.
Enable Multi-Factor Authentication
Prepare users for secure registration and additional sign-in verification.
Review Legacy Authentication
Identify old applications and devices that may not support modern authentication.
Protect Administrator Accounts
Use separate administrative accounts with strong authentication and limited day-to-day use.
New cloud accounts should be protected before they become active and accessible from the internet.
DNS and mail flow
Plan every DNS change before cutover.
DNS records control where email is delivered and which systems are authorised to send messages for the domain.
MX Record
Directs inbound email to Microsoft 365 after the cutover.
SPF Record
Identifies approved systems that may send email for the domain.
DKIM
Adds cryptographic signing to supported outbound messages.
DMARC
Defines how receiving systems should handle authentication failures and reporting.
Autodiscover
Helps supported Outlook clients locate the correct Microsoft 365 service.
Additional Records
Teams, device management and verification may require additional DNS entries.
Before changing DNS
- Confirm who controls the domain DNS.
- Record the existing DNS configuration.
- Reduce relevant TTL values before cutover.
- Identify every legitimate email sender.
- Prepare the final Microsoft 365 records.
- Confirm rollback information.
Migration method
Choose a method that matches the source environment.
The correct method depends on the existing email platform, mailbox quantity, data volume, coexistence requirements and available migration tools.
IMAP Migration
Commonly used to move email messages from compatible mail servers. It may not migrate calendars, contacts or tasks.
Exchange Migration
Used where the source environment is Microsoft Exchange and broader mailbox data must be preserved.
Migration Tool
A specialist tool may support additional data types, reporting, throttling and batch control.
Manual Export and Import
May be suitable for limited cases but becomes difficult to control at scale.
Test the migration method with a pilot group before moving the entire organisation.
Migration batches
Move users in controlled groups.
Batches make it easier to monitor data movement, identify problems and provide focused user support.
Pilot users
Select a small group representing different mailbox sizes, devices and work patterns.
Low-risk departments
Move teams that can tolerate minor adjustments while the process is refined.
High-priority users
Migrate executives, finance and operationally critical users with dedicated support.
Shared services
Move shared mailboxes, distribution lists and application accounts with confirmed ownership.
Cutover and testing
Validate more than mailbox access.
A migration is not complete simply because users can open Outlook. Mail flow, mobile devices, shared access, applications and historical data must also be tested.
Cutover validation checklist
- Confirm inbound email delivery.
- Confirm outbound email delivery.
- Test internal and external messages.
- Verify historical mailbox data.
- Test shared mailbox access.
- Test calendar and contact availability.
- Confirm Outlook profile configuration.
- Confirm mobile-device access.
- Test scanners, websites and applications.
- Review SPF, DKIM and DMARC alignment.
The team should know how to restore mail flow or continue business communication if the cutover does not proceed as expected.
User readiness
Communicate before users are affected.
Many migration issues are caused by unclear instructions rather than failed technology.
Publish the migration schedule
Tell users when their mailbox is moving and what interruption may occur.
Provide sign-in instructions
Explain usernames, temporary passwords, MFA registration and first sign-in.
Explain device changes
Tell users whether Outlook profiles or mobile email accounts must be recreated.
Provide support channels
Give users one clear place to report problems during and after cutover.
Post-migration review
Complete the work after mail flow is stable.
- Confirm all migration batches completed.
- Review failed and skipped items.
- Remove obsolete forwarding routes.
- Secure or decommission the old platform.
- Complete MFA registration.
- Confirm retention and backup requirements.
- Review administrative roles.
- Document the final tenant and DNS configuration.
Final recommendation
Treat the migration as a business-change project.
Email affects customers, suppliers, finance, operations and internal communication. Planning should therefore include technical, security and user-readiness work.
A controlled discovery, pilot, migration and validation process reduces disruption and protects business continuity.