Microsoft 365 Business Mailboxes Move to 100 GB: What to Check

The primary mailbox quota moves from 50 GB to 100 GB for Microsoft 365 Business Basic, Business Standard and Business Premium. Technically Microsoft did not change the existing Exchange plan: it added a companion service, EXCHANGE_STORAGE_50GB, which contributes the extra 50 GB. Both must be enabled on the user for the mailbox to actually reach 100 GB.

The change is part of the 2026 Microsoft 365 packaging updates. The rollout runs from June to September 2026 in successive waves. Each tenant receives a Message Center post before the change lands. That progressive delivery is why two similar companies on the same subscription do not see the same thing at the same time.

The most common case is a companion service that was never provisioned on the user, often after an earlier licence change. The mailbox keeps its original quota even though the tenant received the entitlement. Another case: the service quota did move to 100 GB, but a custom limit set long ago by an administrator on the mailbox itself takes precedence.

Start by confirming that EXCHANGE_STORAGE_50GB is enabled on the affected user in the admin centre. If it is pending, reprocessing the licence usually provisions it. If the service is active but the mailbox is still capped, check the limit set directly on the mailbox: a custom value inherited from an old policy has to be brought back in line with the new quota. These operations run in PowerShell and require Exchange administration rights.

Beware of a common shortcut: having 100 GB does not grant Exchange Online Plan 2 features. Advanced archiving and the related compliance capabilities remain tied to that licence. If your need is raw storage, the Business licences are now enough. If your need is archiving for legal retention, the licensing question is still open.

A saturated mailbox is almost always a symptom of something else: heavy attachments emailed around instead of shared from SharePoint or OneDrive, no archiving policy, or shared mailboxes used as storage. Doubling the quota buys time without addressing the cause. It is a good moment to review how files actually move around the company.