Microsoft has begun increasing mailbox sizes for many Microsoft 365 Business plans, including Business Basic, Standard and Premium. These mailboxes are moving from 50GB to 100GB, giving organisations more room to store emails, attachments and years of communication history.
For business owners and staff, this sounds like a straightforward improvement. More space means fewer warnings about full mailboxes and less time spent deleting old messages. However, there is an important detail that Microsoft hasn’t changed — and it affects how Outlook behaves on your computer.
Outlook still has a 50GB limit
Even though your mailbox in the cloud may now be 100GB, the Outlook app on your computer still stores a local copy of your email in a file called an OST. This file helps Outlook work smoothly offline and keeps everything in sync.
The problem is simple:
Outlook’s OST files become unreliable once they grow beyond roughly 50GB.
This can lead to:
– Slow performance
– Freezing or unresponsive behaviour
– Search results not appearing
– Emails taking longer to send or receive
– Outlook needing to rebuild the entire mailbox file
Rebuilding an OST file can take hours, especially for busy staff or remote workers.
Why doesn’t Microsoft fix this?
The technology behind Outlook’s local mailbox file is old and wasn’t designed for today’s large mailboxes, shared mailboxes or heavy email usage. Increasing the mailbox size on Microsoft’s servers is easy. Updating Outlook’s local storage system is not.
Instead of redesigning Outlook’s storage engine, Microsoft is focusing on New Outlook, which works differently and doesn’t rely on large OST files at all.
What this means for your business
If your organisation uses Outlook on Windows, you may not immediately notice the mailbox size increase — but you might notice Outlook slowing down over time if your mailbox grows too large.
To keep things running smoothly:
– Keep only recent mail offline (3–12 months is usually enough)
– Use Online Archive for older messages
– Avoid caching shared mailboxes unless necessary
– Consider using New Outlook for staff with very large mailboxes
These steps help prevent Outlook from becoming sluggish or unstable.
The bottom line
The increase to 100GB mailboxes is a welcome upgrade, but Outlook’s local storage hasn’t changed. Businesses should be aware of the potential performance issues and take simple steps to avoid disruption.