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You can reduce the risk of losing business email data by choosing a migration path that explicitly covers every data type you need, testing it with a representative mailbox, checking what moved and what did not, and keeping the old service available until users have verified the results. “Email migration” does not always include contacts or calendars: Microsoft’s IMAP migration is email-only, while its documented Google Workspace-to-Microsoft 365 batch process can move email, contacts, and calendars.
Choose a migration route that matches your source, destination, and data
There is no universal business-email migration tool or procedure. Start by naming the exact source and destination platforms, then confirm coverage for mail, contacts, and calendars in that direction. Google lists separate import routes for Exchange Online, other Google Workspace accounts, and IMAP providers; its organization data import guidance points administrators to a migration product matrix for other sources.
| Route | Documented data coverage | Important boundary |
|---|---|---|
| Google Workspace to Microsoft 365, using Microsoft’s Exchange admin center batch migration | Email, contacts, and calendars, according to Microsoft’s migration overview. | Some details do not transfer; see the route-specific limitations below. The batch process supports moving users in groups while both environments remain active. |
| IMAP source to Microsoft 365 | Email only, according to Microsoft’s IMAP migration documentation. | Contacts and calendar data are not migrated by this route; arrange separate supported transfers if you need them. |
| Outlook profile or PST to Google Workspace, using GWMMO | Email, calendar, and personal contact data, according to Google’s GWMMO overview. | The documented workflow has Outlook desktop and Windows requirements. Specific records and settings can be omitted or transformed. |
| Exchange Online to Google Workspace | Google documents a separate Exchange Online migration path; its coverage and omissions are specific to that tool and direction. | Google announced that Workspace Migrate for Exchange Online will no longer be available starting October 2026 and directs administrators to its advanced data import tool. Because that date has arrived, confirm current availability and the supported alternative before beginning, using Google’s Exchange Online migration guidance. |
For a Google Workspace-to-Microsoft 365 move, Microsoft recommends its automated Exchange admin center batch migration over the manual batch approach. Its documentation also says very small businesses may use a different route, such as PST import; the appropriate choice depends on the size and requirements of the project. Microsoft recommends considering help from Microsoft or a partner when planning a Workspace migration.
Inventory what must arrive in the new system
Before selecting a tool, list the accounts and data that matter. This inventory makes it easier to choose a route, design a pilot, and identify exceptions instead of discovering them after users have switched.
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- Mail: Record mailbox and folder structure, including sent items, drafts, junk, deleted items, archives, and attachments. Confirm which folders and message states the selected route includes.
- Contacts: Separate personal contacts from organization-wide address lists, shared contacts, contact groups, and distribution lists. Migration tools may treat these differently.
- Calendars: Identify primary and secondary calendars, shared calendars, resource calendars for rooms or equipment, recurring meetings, attendee responses, time zones, reminders, descriptions, attachments, and permissions.
- Business configuration: Note aliases, delegation, shared mailboxes, devices and apps that send mail, and any identity or domain changes. Decide what must transfer and what can be recreated manually.
- Archives and continuity: Record archive requirements and acceptable interruption. Check whether the source and target can remain available together during the transition.
Do not treat event data and calendar access as the same thing. A migration may copy events without carrying over the sharing permissions that let colleagues see or manage a calendar.
Prepare and test before moving everyone
- Confirm prerequisites for the exact route. Prepare destination accounts, licenses, identity mappings, domains, and required permissions. Microsoft warns that its Google Workspace migration prerequisites must be completed for the migration to succeed; follow the current instructions for your tenant and direction in the relevant Microsoft migration procedure.
- Run a representative pilot. Choose a mailbox with a mix of ordinary and edge-case content: large attachments, nested folders, contacts with several email addresses, shared calendars, and recurring events. Compare the source and destination records, noting omissions and transformations. This is a practical way to expose route-specific issues, not a universal provider-mandated test protocol.
- Agree on a cutover plan. Decide when users will begin working in the destination, who will monitor exceptions, and how new mail and calendar changes will be handled during the transition. Where the chosen process supports batches, move users in groups rather than assuming every account must switch at once.
- Keep both environments usable where the route allows it. Microsoft’s Google Workspace batch process supports keeping both environments active and moving users over time. Confirm how your own configuration handles new mail and changes made during migration.
Run the migration, then verify each data type
Use the migration console and reports provided for the selected route. A successful batch or completed import is not proof that every important record arrived. Microsoft instructs administrators to check whether email, contacts, and calendars migrated after a successful Google Workspace batch. Google’s GWMMO workflow provides migration monitoring and imports contacts first, then calendar data, then email.
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- Email: Check representative old and recent messages, sent mail, folder hierarchy, attachments, and any archive folders in scope. Investigate failed or skipped records in the migration report.
- Contacts: Compare important contacts and their multiple addresses; check organization-wide lists and groups separately if they are part of the project.
- Calendar: Review recurring meetings, time zones, event details, attendee information, and access to shared calendars. Confirm whether users need to recreate permissions.
- Delivery and sending: Test new inbound and outbound mail at the destination. Check aliases and devices or apps that send on behalf of the business. DNS and MX steps depend on the provider and tenant, so use the current instructions for the exact setup rather than assuming one cutover sequence fits every migration.
- Exceptions: Assign an owner to resolve errors and skipped records, and have users verify their own high-priority mail, contacts, calendar events, and shared access.
Know what may be omitted or changed
Google Workspace to Microsoft 365
Microsoft’s documented Google Workspace route does not migrate shared calendars or event colors, and it migrates no more than three email addresses per contact. The largest migratable email message depends on the transport configuration, so check the current detailed procedure for the tenant rather than relying on a single universal size limit. These route-specific caveats are described in Microsoft’s migration procedure.
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Google documents several transformations for this route: contact folders are combined rather than recreated, contact notes larger than 16 KB are truncated, and shared-calendar permissions are not migrated. In the documented Outlook 2016 case, event descriptions can be converted to plain text. Messages whose total size with attachments is larger than 25 MB, or that contain blocked attachment types, may be skipped. Check Google’s current GWMMO migration details and data selection guidance for the precise behavior and requirements.
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Google also warns that a full import into an account that already contains data duplicates contacts and overwrites calendar data. Its “new data” option avoids those particular effects and is intended to resume or add newly arrived data. Choose the import mode deliberately and review Google’s instructions for choosing Outlook data to import before running it against a populated account.
Exchange Online to Google Workspace
Google’s Exchange Online documentation describes omissions that differ from the routes above. Examples include categories, in-place archive mailboxes, certain message states, and email messages larger than 50 MB. Calendar omissions include event attachments, calendar-specific settings, shared-calendar access for non-owners, and events before the migration start date. Treat these as examples for that specific tool and direction, not general rules for email migration. See Google’s Exchange Online migration details and verify that the documented tool remains available for a new project.
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Keep the old service until the move is proven
Do not cancel or decommission the source just because the transfer started or a batch reported success. Keep source mailboxes and exports accessible while users verify important records and administrators resolve reported exceptions. Retire the old service only after the destination is receiving and sending mail correctly, the required data has been checked, and any access that must be recreated has been restored.
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