How to find and export your Outlook data.
Mailbox sizes live in the Microsoft 365 admin center under Reports > Usage, in the Exchange Mailbox usage report. Global Administrators, Exchange Administrators and Reports Readers can open it. Sum the Storage used (MB) column, add archive mailboxes, and report the total in GB.
Where Outlook shows how much you have.
- Sign in to the Microsoft 365 admin center as an Exchange Administrator, Reports Reader or Global Administrator.
- Go to Reports > Usage, select Exchange under Reports, open the Email activity tab, select View More, and choose Exchange > Mailbox usage.
- Select Export to download the per-mailbox table as a CSV. It covers both User and Shared recipient types. Sum Storage used (MB) and divide by 1,024 for GB.
- The report does not include archive mailboxes. Filter on the Has Archive column, then in Exchange Online PowerShell run Get-EXOMailboxStatistics -Identity user@yourdomain.com -Archive for each one and add the sizes it returns.
- For former staff, list inactive mailboxes in the Microsoft Purview portal under Solutions > Data Lifecycle Management > Policies > Retention policies > Inactive mailbox, or run Get-Mailbox -InactiveMailboxOnly -ResultSize Unlimited in Exchange Online PowerShell.
- To spot check one mailbox, open the Exchange admin center, go to Recipients > Mailboxes, select the mailbox, and read Mailbox Usage in the General section.
Plan and role. Any Microsoft 365 plan that includes Exchange Online. Usage reports need a role such as Global Administrator, Exchange Administrator or Reports Reader, and user names are hidden until a Global Administrator turns off the concealment setting in Org settings.
What to report on the Polyshares intake.
Report Outlook in GB. The intake also asks how many years the company has used it and how many seats it has.
Report primary plus archive mailbox storage in GB. The report also has a Deleted Item Size (MB) column for recoverable items; leave it out unless you plan to include held content in the export.
For years in use, the report's Create date column shows when each mailbox was created. Use the oldest create date among long-standing staff. If old mail was imported from an earlier system, count from the year that history starts.
Seats are licensed user mailboxes plus inactive mailboxes of former employees. Shared mailboxes do not need their own license, so include their storage but not as seats. Your license counts are in the Microsoft 365 admin center under Billing and Licenses.
Export the full history.
- Add the person exporting to the eDiscovery Manager role group in the Microsoft Purview portal under Settings > Role groups. The Export role is only in that role group, and a member of Organization Management has to make the assignment.
- In the Purview portal, open eDiscovery > Cases, create a case, and add a search whose data sources are the mailboxes you want, or all people and groups.
- Run the search and select Export. In the tenant-wide source options, turn on the option to include departed users so inactive mailboxes are collected.
- Choose Export items with items report, set the format to Create PSTs for messages, and select Organize data from different locations into separate folders or PSTs to get one PST per mailbox. Each mailbox's primary and archive content merge into that PST.
- Track the export in Process manager and download the packages within 14 days, when they expire. Unzip them with 7-Zip or a similar tool rather than the built-in Windows zip utility.
- For a single mailbox, the user can export from classic Outlook for Windows with File > Open & Export > Import/Export > Export to a file > Outlook Data File (.pst), with Include subfolders checked.
Microsoft says the account running an eDiscovery export needs a Microsoft 365 Enterprise E3 or E5 license. eDiscovery (standard) for email is included in Office 365 E3 and Microsoft 365 E3 and above; premium eDiscovery features need E5 or an add-on.
PST files can be capped at 1, 2, 5 or 10 GB and zip packages at 2, 10, 20 or 40 GB. An export that runs more than seven days is cancelled, so split very large tenants by date range or mailbox group. Items restricted by DLP actions are skipped.
An inactive mailbox only exists if a retention policy, retention label, eDiscovery hold or Litigation hold was on the mailbox before the user account was deleted. Without one, a deleted mailbox is kept for 30 days and then permanently removed.
Outlook PST export works one account at a time on the user's computer. Classic Outlook exports mail, calendar, contacts and tasks with attachments. New Outlook for Windows can export under Settings > Files > Export, but calendar and contact items in PST files are not available there.
The intake only needs the size, so you do not have to run an export to fill it in. You also do not need to clean or scrub an export. Polyshares handles anonymization before anything moves.
Why Outlook data has value for AI training.
Exchange mailboxes hold a company's working record: customer requests, supplier negotiations, internal approvals and escalations, each with sender, recipients and timestamps. Replies and forwards keep the chain intact, so the outcome sits next to the request.
Calendar items in the same mailbox add meeting patterns, and archive mailboxes often reach back many years. Together they show how customers, vendors and internal processes changed over time.
What stays private.
You pick which mailboxes go into the eDiscovery search, so legal, HR or board mailboxes never need to be included.
We anonymize identifying details before anything moves. Polyshares does that work, so your team does not have to scrub records first.
Your company keeps ownership of its data. The license is bounded to defined material and a defined use.
Polyshares is the buyer and pays your company directly. There is no fee or commission, and Polyshares reviews the record before it makes any offer.
Common questions about Outlook data.
Does the Mailbox usage report include archive mailboxes?
No. Microsoft notes that the Storage chart does not include archive mailboxes. Use the Has Archive column to find them and Get-EXOMailboxStatistics with the Archive switch to get their size.
Can we export email from people who left the company?
Yes, if their mailboxes became inactive mailboxes, which requires a retention policy, label or hold before the account was deleted. Inactive mailboxes can be searched and exported with eDiscovery. Otherwise a deleted mailbox can only be recovered for 30 days.
Who can run an eDiscovery export?
Members of the eDiscovery Manager role group, which contains eDiscovery Managers and eDiscovery Administrators. The Compliance Administrator and Organization Management role groups do not include the Export role by default.
How large can an archive mailbox get?
Archive mailboxes start with 100 GB. With auto-expanding archiving on Exchange Online Plan 2, or Plan 1 with Exchange Online Archiving, they can grow to 1.5 TB, so an archive can be much larger than the primary mailbox.
Source: Exchange Mailbox usage report, Export search results in eDiscovery, Create and manage inactive mailboxes. Checked September 2026.
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