Last updated: 21 July 2026
Every Microsoft 365 mailbox has a storage limit built into its plan. Here's what actually happens once a mailbox goes over it — and the fastest way to bring the bill back down.
The baseline
Per-mailbox storage on Microsoft 365 Business Basic, Standard, and Premium — raised from 50 GB as of 1 July 2026.1
Maximum size an auto-expanding archive mailbox can grow to before storage becomes consumption-billed. Included on Business Premium, E3, and E5; an add-on elsewhere.2
Both limits sound generous — until a mailbox has been in daily use for five, ten, or fifteen years, accumulating newsletters, automated notifications, and every attachment anyone has ever sent it. That's the point at which storage stops being free.
Once you're over the limit
Commonly listed at around $0.20 per GB per month (roughly £0.16). A mailbox running 20 GB over its plan costs about £3.20/month on its own — small per mailbox, but this is charged every month, for every mailbox over the limit, indefinitely.3
On Business Basic or Standard, auto-expanding archive (up to 1.5 TB) requires the Exchange Online Archiving add-on at around $4/user/month (~£3.20). It's included free on Business Premium, E3, and E5 — worth checking which plan your organisation is actually on before assuming this cost doesn't apply.2
Once an archive mailbox exceeds the 1.5 TB auto-expand ceiling, further growth is billed as a consumption-based feature at around $0.25/GB/month (~£0.20) — the steepest of the three rates, and the one most likely to apply to long-tenured senior staff mailboxes that have never been cleaned up.2
None of these rates are published as fixed global prices — they vary by region, currency, and licensing agreement. Confirm your tenant's actual rate in the Microsoft 365 admin centre before budgeting against these figures.
Why this compounds
A mailbox that's 20 GB over its limit today doesn't self-correct — it only grows, one newsletter and one meeting-invite attachment at a time. Left alone, the monthly overage bill for that mailbox only ever goes up. The only way the number goes down is for someone — or something — to actually delete the clutter.
Smart Sweep and Storage Cleanup let every employee clear their own clutter in minutes — no per-mailbox IT time, no waiting for a quarterly cleanup project.