Last updated: 21 July 2026
Two native ways to see every mailbox's storage usage across a tenant — no third-party tool needed just to find the problem, only to fix it.
Two native methods
Under Reports in the Microsoft 365 admin centre — storage used, item count, and last activity date for every mailbox in the tenant, exportable to CSV. No scripting required; the fastest starting point for most admins.
Run against Exchange Online PowerShell, looped across every mailbox (Get-Mailbox -ResultSize Unlimited) for a tenant-wide sweep. Returns TotalItemSize, ItemCount, and last logon time — better when you need it scripted, scheduled, or combined with other tenant data.
After the audit
An audit tells you which mailboxes are the problem — it doesn't shrink them. Doing that manually, mailbox by mailbox, is what makes cleanup slow and is why “mailbox full” tickets keep recurring even after an audit identifies the worst offenders. Company-wide self-serve tooling lets each mailbox owner clear their own mailbox directly, rather than IT working through the audit's list by hand.
The Microsoft 365 admin centre's Reports section has a Mailbox usage report covering every mailbox in the tenant — storage used, item count, and last activity date, exportable to CSV. It's the fastest no-scripting option and the right starting point for most admins.
Connect to Exchange Online PowerShell and run Get-MailboxStatistics against each mailbox (or loop it across Get-Mailbox -ResultSize Unlimited for the whole tenant) — it returns TotalItemSize, ItemCount, and last logon time per mailbox. This is the better option when you need it scripted, scheduled, or combined with other tenant data the admin centre report doesn't include.
Manually, mailbox by mailbox, is what makes cleanup slow and is why "mailbox full" tickets keep recurring — see the case for company-wide self-serve cleanup. The audit tells you where the problem is; the actual fix still needs to happen per mailbox, ideally by the mailbox owner rather than IT doing it by hand for each one.
Viewing the Mailbox usage report in the admin centre needs a Global Reader, Reports Reader, or Global Administrator role. Get-MailboxStatistics via PowerShell needs an Exchange administrator role or equivalent — no per-mailbox licence changes required just to audit, only to act on what you find.
Smart Sweep and Storage Cleanup let every employee clear their own mailbox — no per-mailbox IT time working down the audit list.