Find how much you have
Sign in to the Microsoft 365 admin center with a role that can view usage reports, such as Exchange Administrator or Reports Reader. Microsoft lists Global Administrator too but advises using the role with the fewest permissions. Go to Reports, then Usage.
Under Reports, select Exchange, open the Email activity tab and choose View More. In the Email activity drop-down, select Exchange, then Mailbox usage. Pick the 180-day view for the widest window. Microsoft says the data usually covers up to the last 24 to 48 hours.
Click Export to download a CSV of every mailbox. Useful columns include Item count, Storage used (MB), Deleted Item Count, Deleted Item Size (MB), Create date, Last activity date and Has Archive. Shared mailboxes are included as a separate recipient type.
By default Microsoft hides user names in usage reports. A Global Administrator can turn off the Conceal user, group, and site names setting under Settings, Org settings, Services, Reports. Microsoft logs the change to showing names in the audit log. You need names to pick out which mailboxes are in scope.
Total Storage used (MB) and Item count for the in-scope mailboxes and convert to GB. The Storage chart leaves out archive mailboxes, so for rows where Has Archive is true, ask your Exchange admin to run Get-MailboxStatistics with its Archive switch, which returns the archive's size and number of messages. Microsoft recommends the Get-EXOMailboxStatistics version in Exchange Online PowerShell.
For the start of your records, use each mailbox's Create date as a first guess. If mail was migrated in from an older system the true start can be earlier, so ask whoever ran the migration. An eDiscovery search with a date condition also shows statistics with the estimated number and size of matching items. For migrated mail, use the Received or Sent condition, because Microsoft says the Date condition reflects when a message was created or imported from a PST.
What to put on your intake
Your intake has five boxes for each system. Here is what goes in each one for Outlook (Microsoft 365).
- What does it hold?
- Business email in Exchange Online: customer, vendor and partner threads, quotes and bids, approvals and project discussions. Say which mailboxes are in scope, for example: sales, project management, finance and the shared bids inbox.
- Data types
- Text for message bodies. Attachments such as drawings, photos and PDFs are a separate decision, so add Image only if you are willing to license them.
- How big is it?
- Choose GB and enter the total Storage used for the in-scope mailboxes, plus archives if they have them. Put the Item count total in the description and say that the figure includes attachments.
- Verified or estimated
- Verified when it is the Mailbox usage report total for exactly the mailboxes in scope, including archives. Estimated when you scaled up from a few mailboxes or skipped archives.
- Records start
- The date of the oldest message in the in-scope mailboxes. Create date is the mailbox's creation date, not necessarily its oldest message, so note any migration.
Exporting the full history
If a buyer licenses your records, this is how the full history comes out of Outlook (Microsoft 365).
A buyer export happens only after a license is signed and the scope is agreed: which mailboxes, which years, and whether attachments are in. Separately, take your own copy before offboarding anyone, because Microsoft gives you 30 days to restore a deleted account before its data is permanently deleted.
Use the eDiscovery experience in the Microsoft Purview portal, not older guides. Sign in with an account in the eDiscovery Manager role group, select the eDiscovery card, then Cases, open a case, go to the Searches tab, pick the search for your mailboxes and select Export. For a large scope, run Generate statistics first, since Microsoft says that makes large exports faster.
Choose your options on the Export pane. Mail comes out as PST files or as .msg files. For PSTs you set a maximum size of 1, 2, 5 (the default) or 10 GB, and a zip package size of 2, 10 (the default), 20 or 40 GB. Turning on the option to organize by location gives one PST per mailbox, with the primary and archive mailboxes merged into the same PST.
Follow the export in Process manager, then select the export package and choose Download packages. Microsoft advises the latest Microsoft Edge, allowing pop-ups from Purview, and a tool such as 7-Zip rather than the built-in Windows unzip. An items report in CSV comes with the export, which lets you check what was included against your earlier count.
Search export packages are deleted 14 days after they are created, so download promptly. Microsoft also lists a way to download exports programmatically through the Purview eDiscovery API, and a developer can read usage numbers through the Microsoft Graph reports API.
Under the license, names, addresses, phone numbers, account numbers and other identifiers are removed from message text before anything reaches a buyer, and attachments are excluded unless a specific package is agreed.
We only ask for an export after you sign a license and agree the scope. Sizing your data for the intake never needs one.
Limits to know
- Older instructions no longer work. Microsoft retired the classic eDiscovery and Content Search experiences, and the separate eDiscovery Export Tool, on August 31, 2025. The classic Content Search page now says it applies only to Microsoft 365 operated by 21Vianet in China. Everyone else uses the eDiscovery experience in the Purview portal.
- Microsoft's export page says to export content in eDiscovery the user needs a Microsoft 365 Enterprise E3 or E5 license. It does not discuss Business plans, so check with Microsoft or your reseller before assuming the option is open to you.
- Permissions are narrow. Export belongs to the eDiscovery Manager and Administrator role groups and is not given to Organization Management by default. An eDiscovery Manager can open only the cases they created, while an eDiscovery Administrator can open any case.
- Export limits per Microsoft: 2 TB from a single search and 2 TB per organization per day without premium features, 500,000 items and 100,000 locations per export, 5 GB per item, 10 GB per PST, and 40 GB per export package. Mailbox content exports at about 2 GB per hour per mailbox, so a 50 GB mailbox takes about a day. Microsoft cancels any process that runs for 7 days.
- Export packages expire 14 days after creation and the data in them is deleted and cannot be recovered. For search results over 200 GB, Microsoft suggests narrowing with date ranges or other filters.
- Microsoft's indexing limits mark a message partially indexed when an attachment is over 150 MB or there are more than 250 attachments. The export screen has separate options for including partially indexed items, so choose one deliberately or your item count may differ from the usage report.
- The Mailbox usage report has gaps. It shows 7, 30, 90 or 180 days and its data usually trails by 24 to 48 hours. The Storage chart leaves out archive mailboxes, names are hidden by default, and Microsoft notes the shared mailbox summary chart cannot currently be exported.
- Deleted mail does not last. Items a user deletes from Deleted Items sit in the Recoverable Items folder for 14 days by default, up to 30, then are removed from Exchange Online. Litigation holds and retention policies change that, so ask whether any were ever configured.
- Deleted accounts are recoverable for 30 days, after which the data is permanently deleted. Giving another user access converts the mailbox to a shared mailbox, and Enterprise subscriptions can keep it as an inactive mailbox. Find out which you did for any former employee whose mail is in scope.
This guide isn't legal advice. Check your contracts, and any recording-consent rules that apply, with your own lawyer.
Why buyers value it
Outlook often holds the part of the record that never makes it into your other systems. For a finance team, a broker or a consultancy, that means the approval request, the question about an invoice and the client's reply. For a general contractor or specialty sub, it means bid invitations and responses, RFIs, submittal comments, change-order haggling and owner or subcontractor pushback. A single thread can carry the question, the number and the answer, each stamped with a date.
Buyers building bid-response, approval and project-communication tools want real threads from real work. They are worth more when mailboxes are limited to business use, tied to projects, deals or accounts you can name in your other systems, and carry a long, continuous history.
What stays private
Names, email addresses, phone numbers, street addresses and signatures are removed from message bodies and headers before delivery, along with account, invoice and bank details. Personal mail, HR and legal threads, and mailboxes of staff outside the agreed scope are excluded.
Attachments, including drawings, specs, photos, subcontractor quotes and vendor price lists, are excluded by default because many belong to other companies. Calendars, contacts and Teams chats are not part of an email license unless added by agreement. Your logins, tenant settings and admin role lists are not part of the license.
Common questions
Sources
Vendor documentation, checked . Screens and plans change, so if a step looks different in your account, check with Outlook (Microsoft 365).
- Microsoft Learn: Exchange Mailbox usage report (opens in a new tab)
- Microsoft Learn: Microsoft 365 admin center usage reports overview (opens in a new tab)
- Microsoft Learn: Learn about eDiscovery (opens in a new tab)
- Microsoft Learn: Export search results in eDiscovery (opens in a new tab)
- Microsoft Learn: Limits in eDiscovery (opens in a new tab)
- Microsoft Learn: Assign permissions in eDiscovery (opens in a new tab)
- Microsoft Learn: Get started with Content search (classic retirement notice) (opens in a new tab)
- Microsoft Learn: Use the eDiscovery Export Tool in Microsoft Edge (retirement notice) (opens in a new tab)
- Microsoft Learn: Use the condition builder to create search queries in eDiscovery (opens in a new tab)
- Microsoft Learn: Recoverable Items folder in Exchange Online (opens in a new tab)
- Microsoft Learn: Delete a user from your organization (opens in a new tab)
- Microsoft Learn: Get-MailboxStatistics (opens in a new tab)