Pre-migration assessment

Run a mailbox export from the Exchange Admin Center (EAC) or via PowerShell. The key fields to capture are: UPN, display name, mailbox size (GB), item count, and mailbox type (user, shared, room, equipment).

• In EAC → Recipients → Mailboxes, export the list to CSV.
• Flag mailboxes over 50 GB — these take longer and should be scheduled first.
• Identify shared mailboxes — they need a licence in the destination if they will be full mailboxes, or can be converted to shared post-migration.
• List all room and equipment mailboxes — these are often forgotten.

Distribution lists, mail-enabled security groups, and Microsoft 365 Groups all need to be accounted for. Groups that back Teams are handled as part of Teams migration.

• In EAC → Recipients → Groups, export all groups to CSV.
• Note which groups are mail-enabled security groups (MESG) vs. distribution lists (DL) vs. M365 Groups.
• M365 Groups backing Teams will be recreated automatically during Teams migration — do not duplicate them.

Confirm that the destination tenant has enough licences for every mailbox being migrated. Exchange Online Plan 1 is the minimum for email; SharePoint Online Plan 2 is required if migrating OneDrive over 1 TB per user.

List every source UPN (e.g. [email protected]) alongside its intended destination UPN (e.g. [email protected]). Mismatches here are the most common cause of incomplete migrations.

• Export source users to CSV with their UPN and display name.
• Export destination users to CSV.
• Do a VLOOKUP (or similar) on the username prefix to find auto-matches.
• Manually map any users whose prefix differs between tenants.

Items larger than 35 MB (emails with attachments) or 250 MB (SharePoint files) require special handling. Graph API simple uploads are limited to 4 MB — large items must use resumable sessions. Run a large-item scan before starting so you know what to expect.

Before starting, verify the destination tenant has: accepted domains added for all source email domains, AutoDiscover configured, and MX records documented (you will update these at cutover).