To move business email safely, first confirm that your chosen migration route transfers every data type you need—not just messages. Inventory the source, pilot the transfer, move users in manageable stages where supported, check for omissions, and keep the old service available until important mail, contacts, calendars, and shared access have been verified.
Why the migration route matters
“Email migration” does not always mean a complete move of a user’s work data. Microsoft says its IMAP migration transfers email only, while its documented Google Workspace-to-Microsoft 365 batch route can move email, contacts, and calendars. Google lists different import paths for Exchange Online, other Google Workspace accounts, and IMAP providers. Coverage depends on the specific source, destination, and tool—not just the fact that both services support email.
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Start by identifying the exact source and destination platforms, then compare each candidate route against the records and access your business needs to preserve. Google’s organization migration guidance describes its available routes and points administrators to a migration product matrix for other sources.
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Choose a route that matches your source and destination
| Migration route | Documented coverage | Important qualification |
|---|---|---|
| Google Workspace to Microsoft 365 | Email, contacts, and calendars through Microsoft’s batch migration route. | Microsoft recommends the automated Exchange admin center batch approach over the manual batch approach. It supports moving users in groups while both environments remain active. Microsoft’s overview |
| IMAP source to Microsoft 365 | Email only. | Contacts and calendar data are not migrated by this route; arrange a separate supported transfer if you need them. Microsoft’s IMAP migration guidance |
| Outlook or PST to Google Workspace using GWMMO | Mail, calendar, and personal contacts. | Google documents using an Outlook profile or PST. The workflow has supported Windows and Outlook desktop requirements; confirm that your setup qualifies. About GWMMO |
| Exchange Online to Google Workspace | Google documents migration of email and folders in scope, with specific omissions for email and calendars. | Google’s Exchange Online documentation lists limits such as omitted categories and in-place archive mailboxes, plus calendar omissions. These details apply to this route, not to migrations generally. Google’s Exchange Online migration details |
For very small businesses moving from Google Workspace to Microsoft 365, Microsoft notes that a different approach, such as PST import, may be suitable. For a complex or staged project, Microsoft recommends considering help from Microsoft or a partner. Neither option is mandatory for every cloud-to-cloud move.
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Inventory what users need to keep
Before selecting a tool, record what exists in the source and what must be available in the destination. Include data and access arrangements separately: an event can migrate while its sharing permissions do not.
- Mail: mailbox and folder list, sent items, drafts, junk and deleted items, archives, attachments, and folder hierarchy. Confirm the selected route’s scope instead of assuming every folder or archive is included.
- Contacts: personal contacts, contact groups, organization-wide address lists, shared contacts, and distribution lists. Ask whether groups and folders are recreated or merged.
- Calendars: primary and secondary calendars, shared and resource calendars, recurring meetings, attendee responses, time zones, reminders, descriptions, attachments, and permissions.
- Account setup: aliases, delegation, shared mailboxes, devices and apps that send mail, and the identity or account mapping required at the destination.
- Operational needs: number of mailboxes, archive requirements, acceptable interruption, and which records can be recreated manually if the tool does not transfer them.
Plan the move in stages
- Prepare the destination. Set up accounts, licenses, identity mappings, domains, and required permissions before moving content. Prerequisites vary by route; Microsoft warns that its Google Workspace migration prerequisites must be completed for the migration to succeed. Follow the current instructions for your exact path: Microsoft’s Google Workspace migration procedure.
- Run a representative pilot. Choose a mailbox that includes large attachments, nested folders, contacts with multiple email addresses, shared calendars, and recurring events. Compare source and destination records and note omissions or transformations. This is a practical way to expose route-specific issues, not a universal provider-mandated test protocol.
- Move users in batches when the route supports it. Microsoft’s Google Workspace batch process allows both environments to remain active while users move over time. Keep the source usable during the transition where your setup permits it. Microsoft’s automated EAC procedure
- Monitor progress and exceptions. Review the migration console, logs, and error reports; investigate failed or skipped records instead of treating a completed job as proof that everything arrived. Microsoft instructs administrators to check migrated email, contacts, and calendars after a successful batch. Google’s GWMMO workflow provides monitoring and imports contacts first, then calendar data, then email. Google’s GWMMO migration coverage and behavior
- Check delivery and user access. Test inbound and outbound messages using the destination setup, check aliases and the devices or apps that send mail, and reconcile representative old and new messages. DNS and MX changes depend on the provider and tenant, so there is no single safe sequence for every business.
- Resolve exceptions before retiring the source. Keep the old mailboxes and exports available until users have checked important messages, contacts, calendar events, and shared access. Decommissioning is the final phase, not the signal that a transfer has been verified.
Check for data that may be omitted or changed
Messages and attachments
Limits vary by tool and direction. For the documented GWMMO import to Google Workspace, Google says messages with attachments larger than 25 MB, or with blocked attachment types, may be skipped. Microsoft’s Google Workspace-to-Microsoft 365 documentation says the largest migratable message depends on transport configuration. Do not apply either detail as a universal email migration limit; check the current instructions for your chosen route. Google’s Outlook data import options
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On Microsoft’s documented Google Workspace-to-Microsoft 365 path, no more than three email addresses per contact migrate. In GWMMO, Google documents that personal contact folders are combined rather than recreated and that contact notes larger than 16 KB are truncated. Check organizational contacts separately, since they may need another transfer path. Microsoft’s procedure and limitations
Calendars and sharing
On Microsoft’s Google Workspace path, shared calendars and event colors are not migrated. For GWMMO, Google says shared-calendar permissions are not migrated and can be recreated after the transfer; its documentation also describes a case in which Outlook 2016 event descriptions are converted to plain text. These are route-specific caveats, so test the kinds of events and sharing your users rely on. Google’s GWMMO migration details
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Data already in the destination
For a full GWMMO import into an account that already contains data, Google warns that contacts can be duplicated and calendar data overwritten. Its “new data” option is intended to add newly arrived data or resume without those particular effects. Decide which import mode fits the destination before running a full import. Google’s guidance on choosing Outlook data to import
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Check current availability before an Exchange Online-to-Google project
Google announced that Google Workspace Migrate for Exchange Online would no longer be available starting in October 2026 and directs administrators to its advanced data import tool. Because the announced change period begins this month, verify the tool’s current availability and the supported alternative before planning a new migration; do not assume an older procedure remains available. Google’s Exchange Online migration documentation
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Use a verification checklist before closing the old service
- Confirm that the required mailboxes, folders, sent items, and archives are present.
- Check representative messages and attachments, including any records flagged as skipped or failed.
- Compare personal contacts and groups, and verify important organization-wide or shared address data through its intended route.
- Check recurring and one-off events, time zones, attendees, and any needed shared-calendar permissions.
- Test sending and receiving with the destination, including aliases and business apps or devices that send mail.
- Resolve reported exceptions and have users verify critical records before disabling source accounts.
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