Every admin who has worked on a Microsoft 365 move knows the PST problem. Old email sits in PST files on laptops, shared drives and old backups, and someone has to get it into the new mailboxes without losing anything.
Microsoft gives you two free ways to do this. The Outlook import wizard works for one or two small files, but it is slow and depends on one PC staying on for hours. The PST Import Service handles bigger jobs, but you have to upload files to Azure storage with AzCopy, build a CSV mapping file, and track the job in the Purview portal. It works, but each step is a place where things can go wrong.
Stellar Migrator for Outlook aims to replace that process with a simple wizard. I looked at how it handles a real migration, where it saves time, and where it still falls short.
1. What Is Stellar Migrator for Outlook?
Stellar Migrator for Outlook is a Windows desktop tool that moves archive PST files into:
- Microsoft 365 mailboxes
- Live Exchange Server mailboxes
It runs on your own machine, and data goes straight from that machine to your Microsoft 365 tenant. Nothing passes through Stellar’s servers, which matters for teams with strict data rules.
2. Setting Up a Migration Job
The whole process runs through one wizard, step by step.
Create the job. Give the job a name and choose a single mailbox, multiple mailboxes, or a ready-made CSV file. The CSV option is useful for large batches, but unlike Microsoft’s Import Service, it’s optional.
Add source files. Select the PST or OST files. You can add both types in the same job. The tool shows each file’s name, path and size, so you can check you have the right files before going further.
Choose the target. Pick Microsoft 365, Exchange Server or PST.
Connect to Microsoft 365. There are two options:
- Auto Registration: Sign in with a Global Admin account in the browser, and the tool sets up the app connection for you.
- Manual Registration: Create the app in Entra ID yourself and enter the Application ID, Tenant ID and Client Secret. Security teams that want full control over app permissions will prefer this.
3. Mailbox Mapping: Where It Saves the Most Time
Once connected, the tool loads the mailboxes from your tenant and matches each source file to a mailbox by email address. Matched mailboxes show clearly, and you can fix any mismatch by hand.
A few options here are worth knowing:
- Choose the mailbox type: Send data to the primary mailbox, the archive mailbox or a public folder. Sending old PST data to the archive keeps primary mailboxes small.
- Set a priority: Move important users, such as management, first.
- Pick a destination folder: Place imported data in a separate folder, such as “Old PST Data,” so it doesn’t mix with current mail.
One thing to plan for: the tool does not create mailboxes. All target users must already exist and have a license before you start.
4. Filters: Move Only What You Need
Most old PST files carry data nobody needs. The tool lets you filter before migrating:
- Date filter: Move all items, items before or after a date, or items within a date range.
- Folder types: Include or exclude emails, contacts, calendars, tasks, notes, journals, drafts, deleted items and junk email.
Leaving out Junk Email and Deleted Items alone can cut migration time noticeably on older files.
5. Running the Migration
You can run up to 10 mailboxes at the same time. More isn’t always faster, because Microsoft 365 throttles heavy traffic, so start low and increase it if the job runs smoothly.
Before starting, a job summary shows every setting in one place. During the run, you can see overall progress and per-mailbox progress folder by folder.
If something needs to change mid-job, you can update filters, the concurrency limit and expired credentials without starting over. Source and target can’t be changed once the job starts.
6. Incremental Migration: No Duplicates on Re-runs
This is the feature that matters most in real projects. If a job stops partway because of a network drop, an expired login or a restart, you can run it again, and the tool moves only new or changed items. Items already migrated are skipped, so users don’t end up with two copies of the same email.
It also helps when users keep working in the old PST during a phased move. A final re-run picks up whatever arrived since the first pass.
7. Encrypted Files and PST Output
The tool can read password-protected and encrypted PST and OST files, which often stop other tools.
If you choose PST as the target, it can rename output files and split them automatically at about 45 GB, keeping them at a safe size for Outlook.
8. Limitations
No tool is perfect, and these are worth knowing before you buy:
- Login isn’t saved between jobs. You sign in again for every new job, which gets repetitive when you run batches regularly.
- It doesn’t create mailboxes. Users must already exist in Microsoft 365.
- It’s Windows only. There’s no Mac or Linux version.
- You can’t skip attachments. You can filter by date and folder, but not migrate emails without their attachments.
- Speed depends on your setup. It runs from one machine, so the PC, your internet connection and Microsoft’s throttling all affect speed.
9. Pricing
Pricing is per mailbox, and the cost per mailbox drops as volume goes up:
A free demo lets you test it on your own data before buying. Per-mailbox pricing also suits MSPs, who can add the cost to each client quote.
10. When It’s Worth It, and When It Isn’t
Microsoft’s free options are enough if:
- You have one or two small, healthy PST files.
- You have time and someone comfortable with AzCopy and the Import Service.
Stellar Migrator for Outlook makes sense if:
- You’re moving many PST or offline data files for many users.
- The files are spread across different machines and network paths.
- You need to filter data, send it to archive mailboxes or set priorities.
- You can’t afford duplicates if a job has to be re-run.
- You’re an MSP that does PST migrations regularly.
Final Verdict
Stellar Migrator for Outlook handles the PST to Office 365 import job well. It turns Microsoft’s multi-step PST import into a single wizard, and the mapping, filters and incremental re-runs remove most of the manual work.
The main drawbacks are re-entering login details for each job and having to create mailboxes in advance. Neither affects the migration itself.
For a single small PST, Microsoft’s free tools are fine. For anything bigger, especially multi-user projects or MSP work, this tool is worth testing with the free demo.
By Glenn Blake