Last updated: 21 July 2026
What unmanaged mailboxes actually cost, what a MailBroom for Business plan actually costs, and how deletion works alongside the retention policies and legal holds you already have in place.
Cost of inaction
Included per mailbox on Business Basic, Standard, and Premium plans — raised from 50 GB as of 1 July 2026. Anything a mailbox holds beyond its plan's limit is billed as extra storage.1
Typical extra-storage add-on rate (~$0.20/GB/month). Not published as a fixed global price by Microsoft — varies by region, currency, and agreement type, so confirm your own tenant's rate in the admin centre.2
Monthly cost of the Exchange Online Archiving add-on for Business Basic/Standard tenants, needed to get auto-expanding archive up to 1.5 TB (included free on Premium, E3, and E5). Beyond 1.5 TB, overage is billed at ~£0.20/GB/month.3
None of this is a one-off charge. Every newsletter, notification, and old attachment left unswept adds to a mailbox's footprint permanently — and that footprint is re-billed every month it sits above the plan's included storage.
Calculate it yourself
Enter your own headcount and average mailbox size to see the effect of cutting mailbox size in half — a typical result once clutter is cleared out.
Find your real average in Microsoft 365 admin center → Reports → Mailbox usage.
Assuming MailBroom reduces the average mailbox by 50% — a typical outcome once newsletters, notifications, and old attachments are cleared out.
One-off, from clearing today's backlog
That's the one-off effort of clearing the backlog itself — it doesn't count the ongoing time back every time someone actually needs to find an email afterwards. Searching a mailbox with a few hundred relevant messages left in it is faster than searching one still carrying years of newsletters and notifications, every single time it happens — not priced above, since there's no reliable published rate for it, but real all the same.
At 5GB, these mailboxes sit inside Microsoft's 100GB quota already — so there's no overage fee to avoid here. That's not the same as “nothing to gain”: Outlook itself starts showing sync pauses around 10GB regardless of your storage quota6, which is why the IT-time figure above still counts for real, and the hours and CO₂ figures above hold regardless of quota — plus cleanup still buys headroom before these mailboxes grow into the next quota ceiling.
26–50 seats plan
The recurring monthly figures alone don't cover the subscription, but the one-off hours your team gets back from clearing today's backlog do. Before counting CO₂ value or compliance risk. From year two on, without a new backlog to clear, that comparison rests on the recurring figures alone.
Recurring every month after: £0/mo storage overage avoided, £0/mo IT time saved.
Storage freed, CO₂, and overage figures are calculated directly from the sourced rates above. IT time saved uses a published average ticket cost7applied to a conservative, clearly-labelled assumption (one avoidable ticket per year, scaled by how much of a mailbox's excess above Microsoft's documented performance threshold6 was actually removed) — not a published statistic itself. Employee time saved assumes 8s per email cleared (the same rate used on MailBroom's own Dashboard) and 2,300 emails per GB10, valued at the ONS median UK full-time hourly wage9. Everything here is illustrative, not a quote.
IT overhead
“My mailbox is full” is one of the most common Microsoft 365 helpdesk tickets — and the fix is almost always the same manual walk-through, repeated per employee, every few months. MailBroom moves that work from IT's queue to a self-service tool the employee runs themselves, in the browser, in minutes.
Employee raises a ticket → IT talks them through Outlook's built-in cleanup or does it remotely → repeats next quarter for the same mailbox.
Employee signs in with their existing Microsoft account and clears their own mailbox with Smart Sweep or Storage Cleanup — no ticket raised at all.
Compliance
When MailBroom deletes an email, it does exactly what Outlook itself does when a person deletes a message by hand: the item moves into the mailbox's Recoverable Items folder, and — if the retention period expires or the item is purged — into the Purges subfolder marked for eventual removal.4
If that mailbox is on Litigation Hold, an eDiscovery hold, or a retention policy your organisation has already configured, Exchange Online preserves everything in the Purges subfolder for the full hold duration — regardless of what deleted it, and regardless of whether the hold is indefinite.5MailBroom has no path around that mechanism, because it isn't a separate system — it's the same Exchange Online mailbox, accessed through the same Microsoft Graph API Outlook itself uses.
In practice: a company with active holds keeps everything it's legally required to keep, exactly as before. A company with no such hold gets exactly what it asked for — genuinely uncluttered, smaller mailboxes.
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