Key takeaways
- Microsoft shows mailbox sizes in the Microsoft 365 admin center under Reports > Usage > Exchange > Mailbox usage, in megabytes, so you total the Storage used (MB) column and divide by 1,024 to report GB.
- The usage report does not count archive mailboxes or former employees, so add the archives with PowerShell and list the inactive mailboxes in the Purview portal.
- A member of the eDiscovery Manager role group exports mailboxes to PST files from the Purview portal; the account needs a Microsoft 365 Enterprise E3 or E5 license, and the packages are deleted after 14 days.
- You don't need to export anything to apply: have your mail size, years of history and mailbox count ready, and the data vendor anonymizes the records before any AI lab sees them.
01
What is Outlook data, and why do AI labs and data vendors buy it?
The short answer
Outlook data is the stored email and calendar history of a company that runs on Microsoft 365: every message its staff sent and received, the meetings they scheduled, and the attachments on both. AI labs (companies that build and train AI models) and data vendors (companies that anonymize business records and package them for those labs) buy a copy because an email thread keeps a request, the replies and the outcome in order, which shows how a business actually gets work done.
What an Outlook mailbox records
Outlook is Microsoft's email and calendar program, and the mail behind it lives in Exchange Online, the mail service that comes with Microsoft 365 business subscriptions. Each employee has a mailbox holding their messages, calendar entries, contacts and tasks. Many also have an archive mailbox, a second mailbox where older mail is moved so the main one stays small.
Added together, a company's mailboxes record its customer requests, supplier negotiations, internal approvals and escalations. Every message carries a sender, recipients and a timestamp, and replies and forwards keep the whole chain attached.
Why an email thread teaches an AI model something
An AI model learns by studying a very large number of examples and finding the patterns in them, and those examples are called training data. Public text shows finished results, such as a published article, but almost none of it shows how a request was handled. An email thread shows the handling itself: a buyer asks for approval, a manager questions the price, the order is confirmed and the delivery date is set.
AI labs now want to train AI agents, software that carries out a job step by step, and an agent learns a job from records of that job being done. A data vendor can turn anonymized threads into worked examples that an agent studies like an answer key, or build a practice copy of a team's workflow in which the agent is scored on each attempt. The full mechanism is in AI training data: why AI companies want your company's data.
Calendar entries in the same mailbox show when meetings took place and who attended. Because archive mailboxes frequently hold many years of mail, the whole set shows how a company's dealings with customers, vendors and staff have shifted over time.
How Outlook fits into a dataset
Outlook is one of several apps in a company's dataset, which usually also includes documents, chat and tickets. Companies are paid $100K to $2M or more per dataset, according to micro1, the data vendor we typically refer companies to. The price covers the whole dataset, not each app on its own, and what your data is worth explains what moves it.
02
Where does Outlook show how much data you have?
The short answer
Microsoft reports mailbox sizes in the Exchange Mailbox usage report, which sits in the Microsoft 365 admin center under Reports > Usage. Global Administrators, Exchange Administrators and Reports Readers can open it. Add up its Storage used (MB) column, add the archive mailboxes the report leaves out, and divide by 1,024 for a total in gigabytes (GB).
How to read your mailbox sizes
Microsoft measures mail by the space it takes, so the steps below end in a size in GB. Each step names the menu you need.
- Sign in to the Microsoft 365 admin center as a Global Administrator, an Exchange Administrator or a Reports Reader.
- Go to Reports > Usage, and under Reports choose Exchange. On the Email activity tab select View More, then pick Exchange > Mailbox usage from the drop-down list, as Microsoft's Mailbox usage report guide describes.
- Select Export to save the per-mailbox table as a CSV file. It lists both User and Shared mailboxes, and a column named Storage used (MB) holds each size. Total that column and divide by 1,024.
- Add the archive mailboxes, which the report does not count. Filter the CSV on its Has Archive column, then run
Get-EXOMailboxStatistics -Identity user@yourdomain.com -Archivein Exchange Online PowerShell for each mailbox that has one and add the sizes it returns. - Add former employees' mailboxes. Find them in the Microsoft Purview portal under Solutions > Data Lifecycle Management > Policies > Retention policies > Inactive mailbox, or run
Get-Mailbox -InactiveMailboxOnly -ResultSize Unlimitedin Exchange Online PowerShell. - To spot-check one mailbox, the Exchange admin center shows its size too: choose Recipients > Mailboxes, click the mailbox and look at Mailbox Usage under General.
Who can open the report, and the hidden names
Any Microsoft 365 plan that includes Exchange Online has the Mailbox usage report. The roles above can open it, but by default Microsoft hides user names and display names in every usage report. A Global Administrator can show them by changing the concealment setting under Org settings.
| Where | Who can open it | What it shows |
|---|---|---|
| Mailbox usage report | Global Administrator, Exchange Administrator, Reports Reader | Size, item count and create date of user and shared mailboxes. |
| Exchange admin center | An admin with access to the Exchange admin center | The size of one chosen mailbox. |
| Purview inactive mailbox list | Compliance administrator account | Mailboxes of people who have left. |
What the report leaves out
The report's Storage chart does not include archive mailboxes, so its total can be well below the real size. An archive mailbox starts at 100 GB and, with auto-expanding archiving, can grow to 1.5 TB, so one archive may be larger than the primary mailbox.
A separate Deleted Item Size (MB) column measures recoverable items, which are messages users deleted but Exchange still holds. Keep that column out of your total unless the export will include held content.
Why the figure can fall short of the all-time total
The report covers mailboxes as they exist now. Mail that users or retention rules permanently deleted is gone, and a deleted mailbox is kept for only 30 days unless a hold was on it first. Mail imported from an earlier email system counts only after it was imported.
03
What should you know about your Outlook data before you apply?
The short answer
Have three facts ready: how many GB your mailboxes hold, how many years they cover and how many mailboxes are in use. They go into the data manifest, the written summary of which apps hold your records, how many years each covers and roughly how much. You don't need to run an export to find them, and applying through micro1, the data vendor we typically refer companies to, shares no data.
The size in GB
The size is the primary mailbox storage plus the archive mailbox storage, in GB. Counting the archives matters, because the usage report leaves them out.
The years of history
The report's Create date column shows when each mailbox was created, so the oldest date among long-serving staff gives the starting year. If old mail was imported from an earlier email system, count from the year that imported history begins.
The number of mailboxes
The mailbox count is the licensed user mailboxes plus the inactive mailboxes of former employees. Shared mailboxes need no license of their own, so their storage belongs in the size, but they are not counted as seats. You can read the license counts in the Microsoft 365 admin center under Billing > Licenses.
What you don't need to do first
You don't need to export anything to apply, and you don't need to clean or scrub an export. The data vendor anonymizes the records before anyone else sees them, and nothing moves until a lawyer-reviewed license is signed. The application, the intro call and each later step are laid out in how to sell your company's data.
04
How do you export your full Outlook history?
The short answer
A member of the eDiscovery Manager role group runs a search over the mailboxes in the Microsoft Purview portal, then selects Export and chooses PST files, one per mailbox. The person exporting needs a Microsoft 365 Enterprise E3 or E5 license, and the download packages are deleted after 14 days. A single mailbox can instead be exported by its user from Outlook for Windows.
Who can start an export
Microsoft Purview eDiscovery is Microsoft's search-and-export tool for legal and compliance work. Its Export permission sits only in the eDiscovery Manager role group, and a member of the Organization Management role group has to add the person exporting to it under Settings > Role groups in the Purview portal.
Microsoft requires the account that runs the export to hold a Microsoft 365 Enterprise E3 or E5 license. Standard eDiscovery covers email from Office 365 E3 and Microsoft 365 E3 upward, while the premium features call for E5 or an add-on, as Microsoft's guide to exporting search results explains.
Exporting every mailbox with eDiscovery: the steps
- Make sure the person exporting belongs to the eDiscovery Manager role group, as set up above.
- Inside the Purview portal, go to eDiscovery > Cases, start a new case, and create a search that points at the mailboxes you want or at all people and groups.
- Run the search and select Export. Under the tenant-wide source options, switch on the option to include departed users, so that inactive mailboxes are collected.
- Pick Export items with items report and the Create PSTs for messages format. Ticking Organize data from different locations into separate folders or PSTs gives each mailbox its own PST, and a mailbox's primary and archive content land together in that one file.
- Watch progress in Process manager and download the packages before day 14, when Microsoft deletes them. Open the zips with 7-Zip or a similar tool rather than the unzip feature that ships with Windows.
Exporting one mailbox from Outlook itself
A user can export their own mailbox from classic Outlook for Windows, which suits a handful of mailboxes but not a company. The user follows File > Open & Export > Import/Export > Export to a file > Outlook Data File (.pst) and ticks Include subfolders. Classic Outlook carries over mail, calendar, contacts and tasks together with their attachments.
The newer Outlook for Windows offers Settings > Files > Export, though its PST files cannot hold calendar or contact items.
What each route covers
The usage report and the eDiscovery export treat the same four things differently, which is why the number you read and the files you receive can disagree. The diagram compares them.
Limits to plan around
The export lets you set a ceiling of 1, 2, 5 or 10 GB per PST file and 2, 10, 20 or 40 GB per zip package. An export that runs for more than seven days is cancelled, so a very large organization should split its export by date range or by group of mailboxes. Items restricted by data loss prevention (DLP) actions, which are rules that block sensitive content from leaving the company, are skipped.
Mail from people who have left
A former employee's mail survives only as an inactive mailbox, which Microsoft creates when a retention policy, retention label, eDiscovery hold or Litigation hold covered the mailbox at the moment the account was deleted. With no such protection, Microsoft holds a deleted mailbox for 30 days and then erases it, as its guide to inactive mailboxes explains. Inactive mailboxes can be searched and exported with eDiscovery.
What to settle before exporting mail
Employees' mailboxes hold personal and customer information, so the company's lawyer should confirm that licensing the mail is allowed under its employment agreements, privacy policies and customer contracts. Is it legal to sell company data covers the questions to ask. Leaving out the mailboxes of executives, legal and HR is a normal way to begin.
How long an export takes
Microsoft gives no standard time, and larger exports take longer. The Process manager page shows a progress bar and an estimated completion time, so start early and keep the 14-day download window in mind.
Sources: Microsoft Learn, Exchange Mailbox usage report, Export search results in eDiscovery and Create and manage inactive mailboxes. Steps checked against these pages in November 2026.
05
What stays private when Outlook data is licensed?
The short answer
The company chooses which mailboxes go into the search, and the data vendor replaces names, email addresses and other personal details with consistent stand-ins before an AI lab sees anything. The company keeps ownership of its mail, and the license covers defined records for a defined use.
You choose what is in scope
Only the mailboxes you put in the eDiscovery search are exported, so legal, HR and board mailboxes never need to be included. The data vendor also deletes records too sensitive to clean safely instead of anonymizing them.
Names are replaced before a lab sees them
Anonymization means replacing every detail that identifies a person, such as a name or an email address, with a stand-in that is the same every time it appears. The thread still reads in order, so a lab can learn from it, but it no longer says who the people are. The diagram below shows one short thread before and after.
The method, including what is deleted outright and how you review samples, is in how company data is anonymized for AI training.
Ownership and payment
Licensing gives the buyer limited rights to a copy, while the company keeps ownership of the originals. The terms that matter, such as exclusivity, permitted use and deletion, are covered in data licensing. micro1 pays in full within 30 days of accepting the data, usually as a single payment.
Questions
Frequently asked questions
Does the Outlook mailbox usage report include archive mailboxes?
No. Microsoft states that the Storage chart leaves out archive mailboxes. Filter the report's Has Archive column to find them, then run Get-EXOMailboxStatistics with the Archive switch to get each size.
Is mail from ex-employees still exportable?
Yes, when their mailboxes have turned into inactive mailboxes, which happens only if a retention policy, label or hold covered the mailbox before the account was deleted. eDiscovery can search and export those. If nothing protected the mailbox, it can be recovered for just 30 days.
Who can run an eDiscovery export in Microsoft 365?
People in the eDiscovery Manager role group, a group that contains both eDiscovery Managers and eDiscovery Administrators. Compliance Administrator and Organization Management members do not get the Export role automatically, although an Organization Management member can assign it.
Which Microsoft 365 license do we need to export mailboxes?
The account running the export needs a Microsoft 365 Enterprise E3 or E5 license. Standard eDiscovery for email is included from Office 365 E3 and Microsoft 365 E3 upward, while premium eDiscovery features need E5 or an add-on.
How large can an Outlook archive mailbox get?
It starts at 100 GB, and auto-expanding archiving, offered on Exchange Online Plan 2 or on Plan 1 when Exchange Online Archiving is added, lets it grow to 1.5 TB. An archive can therefore dwarf the primary mailbox.
Do we need to export Outlook before we apply?
No. The application takes about two minutes and shares no data. You only need your approximate mail size in GB, your years of history and your mailbox count for the intro call, and the Mailbox usage report gives you the first two. The full export is needed later, when the records are transferred after you sign the license.
What is Outlook data worth to AI labs and data vendors?
Email history is priced as part of a whole dataset rather than on its own. Companies are paid $100K to $2M or more per dataset, according to micro1, and the price depends on the years of history and how completely the records show each job. What your data is worth explains the factors.
Bottom line
Know what is in Outlook, then see what it is worth
You don't need to export anything to apply. Apply in two minutes, describe your records on the intro call, and micro1, the data vendor we typically refer companies to, prices the whole dataset and sends an offer 1–2 days later.
Not legal, tax or financial advice. Figures come from micro1's published pages and public reporting as of November 2026.