Your Microsoft 365Microsoft's subscription bundle of email, Office apps and online storage used by many businesses. Business Mailbox Just Doubled to 100 GB. Here's What That Actually Means

If your business runs on Microsoft 365 Business Basic, Business Standard, or Business Premium, there's a genuinely useful change rolling out right now: your primary mailbox limit is doubling from 50 GB to 100 GB, at no extra cost.

Microsoft announced the details in August 2026 as part of its broader Microsoft 365 packaging updates, with the rollout running from June through to September 2026. If you haven't seen the change in your tenant yet, that's normal. It's arriving gradually, and not every business gets it at the same time.

Here's what you need to know, in plain English.

What's changing

Until now, Business plan mailboxes were capped at 50 GB. Enterprise plans have enjoyed 100 GB mailboxes since 2011, so this brings small and medium businesses up to the same level as the big end of town.

The change applies to your primary mailbox. That's your active inbox, calendar, and contacts. It covers users on:

• Microsoft 365 Business Basic
• Microsoft 365 Business Standard
• Microsoft 365 Business Premium

There's nothing to buy, no plan to switchA device that joins multiple computers and devices together on the same network using cables., and no settings to change. Microsoft delivers the extra 50 GB automatically through a licensing update behind the scenes.

Why this matters for your business

Fifty gigabytes sounds like a lot until it isn't. We regularly see mailboxes belonging to business owners, estimators, and admin staff creeping toward the limit, especially in businesses that email large attachments like drawings, photos, quotes, and site documentation.

When a mailbox fills up, email stops arriving. For most businesses that's not an inconvenience, it's lost work. Doubling the ceiling buys meaningful breathing room, particularly for long-serving staff whose mailboxes have been accumulating for a decade or more.

The fine print worth knowing

A few details that catch people out:

The rollout is gradual. Two users in the same business can have different limits during the transition. If someone is bumping up against 50 GB right now, don't assume the increase has already landed for them. It needs to be verified per user.

Custom limits stay in place. If a mailbox has previously been set to a smaller quota deliberately, the new entitlement won't override that. It stays where it was set until someone changes it.

Licences don't stack. If a user holds multiple Microsoft licences, they get the highest single mailbox entitlement, not the total of all of them. Two licences with 100 GB each still means a 100 GB mailbox, not 200 GB.

Shared mailboxes aren't included. Unlicensed shared mailboxes, the kind several staff use for accounts@ or info@ addresses, stay at 50 GB. Raising those requires a specific licenceYour legal permission to use a piece of software, usually paid per user or per device., so they need to be managed differently.

It's storage, not archivingMoving older data you rarely use into long term storage to free up space while keeping it available.. The change doesn't add any email archiving capability. It simply makes the primary mailbox bigger.

What a bigger mailbox doesn't fix

This is the part most announcements skip.

A 100 GB mailbox is not a backupA spare copy of your data kept somewhere safe so you can recover it if something goes wrong.. Microsoft keeps your email service running, but protecting your data is still your responsibility under Microsoft's own shared responsibility model. If an account is compromised, a staff member deletes the wrong folder, or a departing employee cleans out their mailbox on the way through the door, a bigger quota does nothing to help you recover.

A bigger mailbox also isn't a filing system. Plenty of businesses use email as their de facto document store, and doubling the limit makes that habit easier to keep, not better. Important documents, contracts, and project files belong in a proper shared location where the whole team can find them, not buried in one person's inbox.

What to do about it

Three practical steps:

1. Check who's close to the limit now. If anyone in your team is near 50 GB, confirm whether their mailbox has received the increase yet before it becomes a problem.
2. Review your shared mailboxes. If any are approaching capacity, they'll need attention regardless of this change.
3. Make sure your email is actually backed up. More storage means more data at risk if you're relying on Microsoft alone to protect it.

If you'd like a hand checking where your mailboxes sit or making sure your Microsoft 365 data is properly protected, we're happy to take a look.