Moving one PST file into one Office 365 mailbox is simple. Moving 60 PST files into 60 different mailboxes, each one matched to the right person, is a different job altogether. Done by hand, it can eat up days of an admin's time and still leave room for mistakes.
This guide follows a real-world style scenario of an IT admin facing exactly that task, shows why the usual methods struggle at this scale, and walks through how to migrate multiple PST files to different Office 365 user mailboxes in one go.
Why PST files pile up in the first place
PST (Personal Storage Table) files are Outlook's local data files. Over the years, they collect emails, contacts, calendars and tasks, often because of:
- Mailbox size limits that forced users to archive old emails
- Auto-archive settings that quietly created new PST files
- Employees who backed up their mail before changing laptops
- A move from POP3 or an on-premises Exchange server, where mail lived on local machines
When a company moves to Microsoft 365, these files do not go along automatically. Unless someone imports them, years of business communication stay stranded on old laptops, shared drives and external disks.
The scenario: 60 employees, 60 PST files, one deadline
The company in this story is a mid-sized accounting and consulting firm with about 60 employees. For years, the team used Outlook with POP3 mail, so each person's email lived in a PST file on their own computer. The firm decided to move to Microsoft 365 for better collaboration, shared calendars and cloud access.
The IT admin, Neha, set up the 60 user mailboxes in Microsoft 365 over a weekend. The easy part was done. What remained was the harder part: each employee's PST file, ranging from 2 GB to 18 GB, had to land in that employee's own mailbox. If a manager's emails ended up in an intern's inbox, the firm would have a serious privacy problem.
The firm had set a cutover date. After that date, the old PCs would be reformatted and handed out again, so the PST files had to be safely in the cloud before then.
What Neha tried first
Option 1: Importing through Outlook. She opened a few users' profiles and used Outlook's import feature. This works for a single mailbox, but it means logging in to each profile, copying data across and waiting for it to sync, one user at a time. For 60 users with large files, the time added up quickly, and a dropped connection could leave a half-copied mailbox.
Option 2: The Microsoft Import service. Microsoft offers a PST import service in the Microsoft Purview portal. It works through uploading files over the network (or shipping drives in some regions) and then mapping each PST to a mailbox using a CSV file. It is a capable method, but Neha hit practical hurdles:
- She had to collect all 60 PST files in one location and upload them using a command-line tool
- Building and troubleshooting the mapping file took time, and a small typo in a mailbox address could cause an import to fail
- Some PST files were large or had minor corruption, which slowed or interrupted the process
- Progress was hard to monitor, and a retry meant restarting parts of the job
With the deadline approaching, she needed something faster and more controllable.
The solution: Shoviv Outlook PST to Office 365 Migration Tool
Neha looked for a dedicated tool that could handle many PST files in one session and let her map each file to its own mailbox. She tested Shoviv Outlook PST to Office 365 Migration Tool using the free demo before committing. Here is how the migration went.
Step 1: Connect to Office 365
After installing the tool on an admin workstation, she signed in with her Microsoft 365 administrator account. This gave the tool access to the tenant's mailboxes so she could pick destinations for each file.
Step 2: Add all the PST files together
Instead of opening one profile at a time, she added all 60 PST files in a single batch from the shared folder where she had gathered them. The tool read the files and showed their folder structures, so she could see what was inside before moving anything.
Step 3: Map each PST file to the right mailbox
This was the most important step. She matched every PST file to its owner's Office 365 mailbox. For a large number of users, a mapping file in CSV format is the fastest route. She listed each PST file path and its target mailbox, imported the list, and the tool applied it across the whole batch. A quick review of the mapping screen helped her catch two files whose names did not match the usual pattern before anything was migrated.
Getting this mapping right is what separates a clean migration from a privacy incident, so it is worth taking a few extra minutes to double-check.
Step 4: Apply filters and choose what to migrate
The firm did not need every old item. Neha used filters to include only the emails from the last several years and left out the Deleted Items and Junk Email folders. Date and folder filters cut the data volume and shortened the migration time without losing business records.
Step 5: Run the migration and monitor progress
With mappings and filters set, she started the migration. The tool processed multiple mailboxes in parallel, and a progress view showed the status of each one. When a single file hit a network hiccup, she re-ran just that mailbox instead of restarting everything.
Step 6: Verify the results
After the migration, she logged in to a sample of mailboxes, compared item counts and folder structures against the original PST files, and checked that emails, calendar entries and contacts were present. She also asked a few employees to confirm their old emails were where they expected.
The result
By the cutover date, Neha had:
- All 60 PST files migrated to the correct individual mailboxes
- Original folder structures preserved, so users found their emails where they left them
- A clear record of what was migrated for each user
- The old PCs cleared to be reformatted without any risk of lost mail
What had looked like several days of repetitive work turned into a controlled, monitored batch job.
Best practices before you migrate multiple PST files
Collect and inventory your PST files first. Search user machines and shared drives, and list each file with its owner. Missing files are the most common reason users complain after a migration.
Make sure the target mailboxes exist and are licensed. Create the Office 365 mailboxes before you start. Migrating to a mailbox that is not ready leads to errors.
Check PST health. Large or old PST files can have corruption. Scan questionable files first, and repair them if needed, so they do not fail halfway.
Double-check your mapping. Confirm every PST file points to the right user. A misplaced file is far harder to fix than to prevent.
Run a pilot migration. Migrate two or three users first, confirm the results, and then run the full batch.
Use filters to reduce data. Migrating only what users need saves time and storage in Microsoft 365.
Keep the original PST files until you are sure. Do not delete the source files until users have verified their data.
When does a dedicated tool make sense?
Manual or built-in methods can work for a few mailboxes. A dedicated tool is worth considering when:
- You are migrating dozens or hundreds of PST files
- Each PST must go to a different user mailbox
- Files are large, old or partly damaged
- You need filters, progress tracking and the ability to retry failed items
- The cutover deadline leaves little room for trial and error
Final thoughts
Migrating PST files to Microsoft 365 is less about the technology and more about doing it accurately at scale. The right preparation, a careful mapping and a way to monitor the job make the difference between a smooth cutover and a week of support tickets.
If you are planning a similar migration, you can download the Shoviv Outlook PST to Office 365 Migration Tool demo to try the process with a small set of files before running your full batch.
Frequently asked questions
Can I migrate multiple PST files to different mailboxes in one go? Yes. A migration tool lets you add many PST files, map each to a different Office 365 mailbox and run them together as one job.
How do I make sure each PST goes to the right user? Use a mapping list that pairs each PST file with its target mailbox, and review it before starting. A small pilot run is a good extra check.
Can I migrate PST files into archive mailboxes instead of primary mailboxes? In many cases, yes. If your goal is to keep old emails out of the main mailbox, check that the tool and your Microsoft 365 plan support archive mailbox migration.
What happens if a migration is interrupted? A good tool lets you re-run the affected mailbox without restarting the whole job. Always confirm item counts after a retry.
Do I need Outlook installed on the machine? Requirements vary by tool. Check the system requirements on the product page before you begin.
Will the folder structure be preserved? A reliable migration keeps the original folder hierarchy, so users find their emails where they expect them.
Read more: https://apps.microsoft.com/detail/xp9m2w9hv4dq1g