Google Workspace Migration for Microsoft Outlook, usually shortened to GWMMO, is Google's free tool for moving mail, contacts, and calendar data out of Outlook and into a Google Workspace account. It works well for a single admin moving a handful of mailboxes, but it was built as a desktop utility, not a batch platform. GWMMO holds up reliably across roughly one to twenty accounts. Push past that range, or feed it a damaged PST file, and you start risking partial transfers, dropped items, and migrations that stall out partway through.
If you're planning a move for a single person or a small office, GWMMO is a reasonable place to start. If you're coordinating dozens of mailboxes as part of a broader rollout, it's worth knowing where that free tool runs out of road before you commit a project timeline to it.
GWMMO needs a few things in place before it will run cleanly. Confirm each of these before you open the installer.
You need valid Google Workspace admin credentials for the destination account. GWMMO authenticates against that account directly, and without the right permissions it can't authorize itself to write mail, contacts, or calendar data into the mailbox you're targeting.
You need access to the machine that holds the Outlook profile or the exported PST file, since GWMMO runs locally and reads from that file on disk rather than connecting to a remote server.
You need a PST file that's actually intact. A corrupted or badly oversized PST file is one of the more common reasons a GWMMO run stalls or drops content partway through. If the source mailbox has a history of sync errors, corrupted folders, or unusual size, it's worth running an Outlook repair pass or exporting a fresh PST before you start the migration, rather than finding out mid-run.
Finally, confirm the Google Workspace account you're migrating into already exists and is provisioned with the right licensing and storage. GWMMO doesn't create or configure accounts for you. It only moves data into one that's already set up.
GWMMO is a per-user desktop tool. You install it, point it at one PST file or mailbox, run the migration, and repeat the entire process for the next person. That workflow holds up fine for roughly one to twenty accounts. Past that range, the manual repetition and the lack of batch processing start working against you, and the tool's own limitations become more visible: it has no advanced filters for selective migration, it can't handle a corrupted PST file gracefully, and larger accounts are more likely to run into content accuracy issues where some items simply don't land where they should.
Server-side and third-party migration tools solve a different problem. Services like VaultMe, Shoviv, and Cloudiway are built to run batch jobs across many accounts at once, with pre-migration checks that catch blockers before data starts moving, real-time progress reporting, and in some cases a window after the migration completes where source and destination accounts stay in sync so nothing gets missed. VaultMe, for example, supports migrations from hundreds of email providers, not just Outlook, and includes detailed per-user migration reports along with dedicated support if something goes wrong. These are paid services, and the tradeoff is straightforward: you give up GWMMO's zero cost in exchange for a tool built to handle the account volume and the troubleshooting workload that a desktop utility was never designed for.
The practical decision point: if you're migrating a small office or a single department, GWMMO's cost and simplicity make sense. If you're coordinating migration across dozens or hundreds of users, a server-side approach reduces both the manual workload and the error risk that comes with running GWMMO that many times. Suitebriar's Google Workspace onboarding and migration work is built around exactly this kind of larger-scale project, pairing the technical migration with the change management that keeps end users oriented once their mailboxes move. Results depend on the specifics of each environment, so any timeline or scope should come out of an actual assessment rather than an assumption based on account count alone.
Running GWMMO follows a consistent four-step sequence: download and install the tool, authorize it against the destination Google Workspace account, export the source Outlook data to a PST file, and then select that file along with the content types you want to migrate before starting the run. Each step depends on the one before it, so it helps to work through them in order rather than skipping ahead.
Download the GWMMO installer from Google and run it on the machine that has access to the Outlook profile or the PST file you plan to migrate. GWMMO runs locally, so it needs to sit on the same machine as your source data rather than on a separate server.
During installation, the tool will ask for a set of local permissions to read Outlook data and write to disk. Grant those permissions when prompted. If you're installing on a shared or managed machine, confirm with IT that the installer has the local admin rights it needs, since a partial installation is a common source of confusing errors later in the process.
Once GWMMO is installed, launch it and sign in with the destination Google Workspace account, not the source Outlook account. This is the step where the tool gets permission to write mail, contacts, and calendar data into that mailbox, so take a moment to confirm you're signing into the correct account before granting access.
This matters more than it might seem. If the wrong account gets authorized, even briefly, data can end up landing in a mismatched mailbox instead of the one you intended. Double-check the account name on screen before you approve the permissions GWMMO requests.
GWMMO imports from a PST file, so the next step is exporting the source Outlook mailbox or Exchange account into that format. Use Outlook's own export function to create the PST archive, and choose a location on disk that GWMMO can easily reach once you point it at the file.
Before you move on, confirm the resulting PST file isn't corrupted. A damaged PST is one of the more reliable ways to have GWMMO fail partway through an import, and it's much easier to catch a bad export now than to troubleshoot a stalled migration later. If the file looks unusually small given the mailbox size, or Outlook threw any warnings during export, re-run the export before proceeding.
With the PST file ready, open GWMMO and select that file as your source. The tool will let you choose which content types to bring over: mail, contacts, and calendar entries. Select the ones relevant to this account and leave out anything you don't need migrated.
Click Migrate to start the job, then let it run to completion before clicking OK to confirm. GWMMO copies items rather than moving them, so the original data stays in Outlook throughout the process. That's worth knowing going in, because it means you'll have the source mailbox available afterward to compare against what landed in Google Workspace, which is exactly what you'll want for the reconciliation step later.
GWMMO doesn't batch accounts. Every additional user means repeating the full sequence from installation through authorization through PST export and migration, one person at a time. For a handful of mailboxes that's a manageable afternoon of work. For a few dozen, it turns into a significant time commitment, and the repetition itself introduces more chances for a mistyped account, a missed permission, or a corrupted file to slip through unnoticed.
This is the practical signal that it's time to consider a server-side or third-party tool instead of continuing to run GWMMO user by user. Tools built for batch migration handle many accounts in a single setup, run pre-migration checks across all of them at once, and surface problems before they turn into a failed run. If you're still inside that roughly one-to-twenty account range, GWMMO's manual approach remains workable. Past it, the time saved by a batch tool tends to outweigh the fact that it isn't free.
A completed GWMMO run isn't the same as a confirmed migration. Before you consider a mailbox done, check item counts against the source and look for any leftover sync behavior from a run that didn't finish cleanly. Both issues show up more often than admins expect, and both are straightforward to catch if you check for them.
Compare the number of messages, contacts, and calendar events in the original Outlook PST or mailbox against what actually landed in Google Workspace. This matters most on larger PST files, where content accuracy issues are more likely to show up during a GWMMO run than they are on a small mailbox.
If the counts don't match, don't assume the difference is cosmetic. Missing items usually mean a re-run of the affected folder or a manual recovery of whatever didn't transfer. Since GWMMO copies rather than moves data, the originals are still sitting in Outlook, so you have what you need to track down exactly what's missing and pull it over manually if a full re-run isn't practical.
One documented case on a Microsoft Q&A forum describes an admin who installed GWMMO, canceled the migration partway through, and afterward noticed that emails sent from their Google account were also appearing in the Sent Items folder of their Microsoft Exchange account. The poster mistakenly installed GWMMO (Google Workspace Migration for Microsoft Outlook) and canceled the process midway, and ended up with a lingering connection between the two accounts that kept duplicating sent mail. The thread doesn't record a confirmed fix, so treat this as a reported case rather than a guaranteed outcome if you run into something similar.
Part of the confusion in that case traces back to mixing up two different tools. GWMMO is a one-time migration utility: it copies data once and is meant to finish. GWSMO, Google Workspace Sync for Microsoft Outlook, is a different tool built for ongoing sync between the two platforms rather than a single transfer. If you cancel a GWMMO run partway through and later notice mail duplicating or syncing unexpectedly, check whether a sync connection was left active, and disable it if so. That leftover connection, not GWMMO itself, is the likely source of the ongoing duplication.
A migration is ready to call done once your reconciled item counts match between source and destination and there's no active sync connection left running in the background. Confirm both before you move on to the next mailbox, since an unresolved sync link is exactly what caused the duplicate Sent Items problem described above.
Once a mailbox is confirmed complete, look at what's left on your list. If you're down to a few remaining users, continuing with GWMMO one account at a time is still reasonable. If you're facing a larger group of mailboxes or accounts with unusually large PST files, that's the point to weigh a server-side migration instead of repeating the manual process again and again. Suitebriar, as a Google Cloud Premier Partner and Google Workspace reseller, supports exactly that kind of transition, pairing the technical migration work with the change management and user adoption support that larger rollouts need. Any specific scope or timeline for that kind of project should come from an actual assessment of your environment rather than a general estimate.