Microsoft 365 Mailboxes Are Moving to 100 GB – But Check Yours

Microsoft is increasing the primary mailbox capacity available with eligible Microsoft 365 Business plans from 50 GB to 100 GB. That is welcome news for any business where inboxes are filling up. But there is a catch: the change is being rolled out gradually. Some organisations already see 100 GB mailboxes, while others — sometimes within the same organisation — still see 50 GB.

The practical advice is simple: check before buying anything. If a mailbox is running out of space, the right first step is not to purchase an extra licence or an add-on. It is to find out what your organisation is already entitled to, and why the mailbox has not picked it up yet.

What is changing?

For years, Microsoft 365 Business mailboxes have been limited to 50 GB of primary mailbox storage. Microsoft is now introducing an additional 50 GB of primary mailbox storage for eligible Business licences, bringing the potential total to 100 GB per mailbox.

• The additional capacity is being provisioned gradually, tenant by tenant and mailbox by mailbox.
• Different users within the same organisation may temporarily show different limits.
• When the additional service becomes active for a mailbox, the new limit should eventually apply automatically.
• Not every mailbox shows 100 GB yet — and that is expected during a staged rollout.

In other words, this is a genuine improvement — but it is not a switch that flips for everyone on the same day. Treat any single mailbox as unconfirmed until someone has actually checked it.

Why might a mailbox still show 50 GB?

There are four common reasons, and they need different responses — which is exactly why guessing is a bad idea.

1. The rollout has not reached that mailbox yet. Microsoft provisions cloud changes in stages. The additional storage service may still be absent or pending for a particular tenant or user. If so, the honest answer is often simply to wait — no purchase required.

2. An old quota remains attached. Some mailboxes have had their storage quota explicitly configured in the past. That older setting can remain in place even after the licence entitlement changes, quietly capping the mailbox at the old limit.

3. The licence or service plan is different. Not every Microsoft 365 or Exchange Online licence has identical mailbox features. An administrator must confirm exactly which licence and service plans are assigned to the user before assuming an entitlement exists.

4. Documentation and tenant state may not update at the same time. Microsoft's public product pages may not yet reflect the rollout consistently. The actual mailbox and licence state should be checked directly, rather than relying on a product summary page.

What should your IT provider check?

Before anyone recommends a licence change, an add-on or a quota edit, these questions should be answered. They normally require Microsoft 365 administrator access:

• Which Microsoft 365 licence is assigned to the user?
• Is the additional mailbox-storage service present on that licence?
• Is its provisioning status active or still pending?
• What quota is currently applied to the mailbox?
• Has a custom quota been set on the mailbox previously?
• Is the mailbox genuinely close to capacity, or is this a false alarm?
• Would an online archive be more appropriate than simply enlarging the primary mailbox?
• Is there a retention or email-management problem causing uncontrolled growth?

Can the quota be corrected?

Sometimes, yes. Where the licence entitlement is already active but the mailbox retains an old quota, an administrator may be able to correct the warning, send and send/receive thresholds using Exchange Online administration tools or PowerShell.

Where the additional storage service is still pending or absent, changing the quota alone may not work — a quota setting cannot create an entitlement that Microsoft has not provisioned. In that situation the administrator may need to wait for Microsoft's rollout, check licence provisioning, reprocess the licence assignment, or raise a Microsoft support case.

Should every mailbox grow to 100 GB?

More capacity is useful, but it does not solve every email-management problem — and in some cases it hides one.

• Large mailboxes can take longer to search, synchronise and manage.
• Old email may belong in an online archive rather than the primary mailbox.
• Retention requirements should be documented, not improvised.
• Important information should not exist only inside one person's mailbox.
• A full mailbox may reveal poor filing, retention or leaver-management processes.
• Additional capacity should not replace backup.

That last point deserves emphasis: mailbox capacity is not backup. A bigger mailbox holds more email; it does nothing to protect that email from accidental deletion, malicious activity or account compromise.

What does this mean for licensing?

Businesses should not cancel Exchange Online Plan 2, archiving or enterprise licences solely because a mailbox now shows 100 GB. Those plans may include capabilities the business is actually relying on, such as advanced archiving, retention, compliance features, legal hold, eDiscovery and other Exchange Online Plan 2 capabilities.

The correct licensing decision depends on the features being used, not just mailbox size. Downgrading to save money on storage that is now included elsewhere can quietly remove compliance protections nobody remembers switching on.

Why business owners should care

• You may already be entitled to more storage — without spending anything.
• You should not automatically pay for an extra licence just to solve a mailbox-capacity issue.
• Your IT provider should check the tenant before recommending an upgrade.
• Mailbox capacity should form part of routine Microsoft 365 administration, not a crisis response.
• Cloud changes still need checking; they do not always apply consistently or immediately.

This is the operational heartbeat idea in practice: a Microsoft announcement is not the same as confirming that the change has actually reached your organisation. Someone still needs to check.

The IT Club View

Microsoft 365 is a cloud service, but that does not mean every change happens instantly or cleanly. A good IT provider should not simply sell another licence when a mailbox reaches 50 GB. They should first establish what the customer already owns, what Microsoft has provisioned, whether the quota is wrong, whether archiving is needed, and whether the mailbox is being managed properly.

The value is not just knowing that 100 GB exists. The value is making sure the right users receive it, without creating unnecessary cost or hiding a wider email-management problem.

Connect-ExchangeOnline

Get-EXOMailbox -Identity user@domain.co.uk -Properties ProhibitSendReceiveQuota |
Select-Object DisplayName, ProhibitSendReceiveQuota, ProhibitSendQuota, IssueWarningQuota

Get-Mailbox user@domain.co.uk |
Select-Object UseDatabaseQuotaDefaults, ProhibitSendReceiveQuota, ProhibitSendQuota, IssueWarningQuota

Set-Mailbox -Identity user@domain.co.uk `
-ProhibitSendReceiveQuota 100GB `
-ProhibitSendQuota 99GB `
-IssueWarningQuota 98GB