If Outlook has started warning that a mailbox is almost full, or if an invoice with a large attachment will not send, you are not alone. Through September 2026 Microsoft has been rolling a storage increase to Microsoft 365 Business Basic, Business Standard, and Business Premium. Eligible primary mailboxes move from 50 GB to 100 GB.
That sounds like the problem is solved. For some owners it is. For others the “mailbox full” message is still there, because the extra space has not reached their account yet, or because years of attachments, calendar clutter, and shared-mailbox copies have already eaten the old 50 GB.
Here is what the change actually means in everyday terms, how to check your size, and how to clean or archive without throwing away records you may need later.
What changed, in plain language
The extra space is for the primary mailbox. That is the inbox, sent items, drafts, calendar, and contacts you use every day.
A few details matter for a small team:
- The increase applies to Microsoft 365 Business Basic, Business Standard, and Business Premium user mailboxes.
- It is rolling out in waves from June through September 2026. One person on your team may already show 100 GB while another still shows 50 GB.
- Standalone Exchange Online Plan 1 mailboxes generally stay at 50 GB unless they sit on a Business suite license.
- The extra 50 GB is not a second mailbox. It does not automatically give you a larger archive box.
- On Business Basic and Business Standard, the built-in archive mailbox is still commonly 50 GB. Business Premium has a much larger archive option.
- Shared mailboxes and resource mailboxes follow different rules. Do not assume they doubled too.
A bigger inbox is helpful. It is not the same thing as keeping mail for taxes, disputes, or “who promised what” two years later. Capacity and record-keeping are different jobs.
How to check mailbox size without becoming an admin
Start with the mailbox that complains the most.
In Outlook on the web
- Sign in at outlook.office.com.
- Open Settings, then General, then Storage.
- Note the used space and the limit shown (50 GB or 100 GB).
In the Microsoft 365 admin center (if you have admin access)
- Go to Users, then Active users.
- Open the person, then the Mail tab.
- Look at mailbox usage and the listed quota.
On the desktop Outlook app the number can lag. Trust the web view if the two disagree.
Write down three things: who is full, whether they already have 100 GB, and whether the heavy mail is in Inbox, Sent, a giant folder of PDFs, or a shared mailbox.
Why a 100 GB mailbox can still feel full
The usual culprits are not thousands of short messages. They are:
- Invoices, contracts, and photos sitting in Sent Items
- “Keep this just in case” folders that were never reviewed
- Recoverable Items and Deleted Items that never emptied
- A shared inbox everyone CCs
- Newsletter tools or old printers that mail a PDF to the same person every day
If the account is still on the 50 GB cap, those files hit the wall quickly. If the account already moved to 100 GB, you have breathing room, but the same habits will fill that too.
Simple ways to clean without deleting important records
Do this in order. Stop as soon as the mailbox can send and receive again.
- Empty what is already trash Check Deleted Items and Junk. Empty them. That is the safest first cut.
- Sort by size, not by date In Outlook, view a folder by size. The top 20 items are often old attachments. Download the files you still need to a shared drive or OneDrive, then remove the attachment from the message or move the whole message to archive.
- Use Online Archive if your plan includes it Many Microsoft 365 mailboxes can move older mail into an In-Place Archive. The messages stay searchable in Outlook. They just leave the primary mailbox. This is the cleanest everyday option when you need the mail later but not in the main inbox.
- Keep a simple retention habit A useful rule for a small office: current year plus last year stays handy in the primary mailbox. Older project mail goes to archive. Do not invent a 14-folder system you will never maintain.
- Watch shared mailboxes separately A shared inbox can look “fine” on each person’s computer while the shared box itself is full. Check that mailbox the same way you checked the user mailbox.
Do not turn on random auto-forwarding to a personal Gmail address to “save space.” That creates deliverability and security problems. If mail flow is already shaky, see Your Domain Name, DNS and Email – The 2026 Survival Guide and Email Security Checklist for Small Businesses.
When cleaning is not enough
Move to a real archive plan when any of these are true:
- You must keep mail for a set number of years
- More than one person needs to search old messages
- You cannot risk someone emptying Deleted Items and losing the only copy
- Invoices and contracts live only in email
- A mailbox is full even after obvious junk is gone
Microsoft’s built-in archive helps day-to-day storage. A separate email archive keeps a copy of mail sent and received across the domain, even if someone deletes it from Outlook. KartHost’s Email Archiving page explains that model in simple terms. It is built for retrieval and record-keeping, not just extra gigabytes.
When a different plan makes more sense
You do not need the most expensive Microsoft license just to stop a full-mailbox warning.
A larger primary mailbox is enough when the person mainly needs room for current work.
A Business Premium-style archive is worth a look when one or two people keep huge project histories and already live in Microsoft 365.
A dedicated archive add-on is the better fit when the whole company needs searchable history, not just a bigger inbox for one user.
KloudEmail or Hosted Exchange can be the cleaner path when only some people need a large mailbox and others need a simple branded address. KartHost already supports mixing mailbox types on the same domain in those products.
If the “full” mailbox is still showing 50 GB in mid-September, the extra space may not have landed on that license yet. Confirm the person has an eligible Business suite license assigned. Do not buy a second Exchange plan only for storage until you know whether the 100 GB increase is already included.
A short checklist for this week
- Check mailbox size in Outlook on the web for anyone who has had a send failure.
- Note whether the limit says 50 GB or 100 GB.
- Empty Deleted and Junk first.
- Download or file the largest attachments, then archive older mail.
- Decide whether you need extra inbox space, searchable long-term records, or both.
- Leave DNS and mail routing alone while you clean storage. Those are separate issues.
More space helps. A little housekeeping helps more. The cleanest option is the one that keeps invoices moving now and still lets you find last year’s contract when you need it.
