Outlook Mailbox Size Limit: How Much Storage You Get (2026)
Your Outlook mailbox storage limit depends on the account type, not the app. A free Outlook.com account holds 15 GB of email; a Microsoft 365 personal or family subscription raises that to 100 GB; and business mailboxes on Exchange Online get 50 GB or 100 GB depending on the plan. This is separate from the attachment limit per email.
The Limits at a Glance
Outlook.com (free + consumer Microsoft 365)
- Free Outlook.com account: 15 GB of email storage
- Microsoft 365 Personal or Family subscriber: 100 GB of email storage (source)
- A separate 5 GB free Microsoft cloud quota covers OneDrive files, photos, and attachments (free accounts only)
Microsoft 365 / Exchange Online (business)
- Exchange Online Kiosk / Office 365 F3 (deskless): 2 GB
- Exchange Online Plan 1, Business Basic/Standard, Office 365 E1: 50 GB
- Exchange Online Plan 2, Microsoft 365 E3/E5: 100 GB (source)
- Shared and resource mailboxes: 50 GB unlicensed, 100 GB with a Plan 2/E3/E5 license
Primary Mailbox vs. Archive vs. the .pst File
These are three different storage pools, which is why the numbers feel inconsistent. Your primary mailbox is the 15 GB / 50 GB / 100 GB quota above. On business plans, an archive mailbox is a separate store: each user starts with 100 GB, and with auto-expanding archiving turned on (Plan 2, E3/E5, or Plan 1 with an Archiving add-on), it grows incrementally up to 1.5 TB.
The classic Outlook desktop app for Windows adds a fourth limit. Its local data files — the .pst (archive/POP) and .ost (cached Exchange copy) — are capped at 50 GB by default, set by the MaxLargeFileSize registry value of 51,200 MB. That cap is configurable in the registry, but Microsoft warns that very large files hurt performance. Hit it and Outlook stops adding to the file even if your online mailbox has room.
How to Free Up Mailbox Space
- Empty Deleted Items and Junk. These count against your quota until purged. Right-click each folder and choose Empty.
- Find and delete the biggest emails. Sort by size or search for large messages — they’re almost always old attachments. See how to find large emails in Outlook.
- Move old mail to an archive or local .pst. On business plans, enable the online archive; on desktop, archive to a local file to offload the primary mailbox.
- Clean up the noise. Unsubscribe, delete newsletters, and clear long threads. Our Outlook inbox cleanup guide walks through it step by step.
- Save attachments elsewhere. Download large files to OneDrive or your computer, then delete the original email.
Troubleshooting
What happens when my Outlook mailbox is full?
You can’t send or receive email. Incoming messages bounce back to the sender, who must resend once you’ve freed up space.
Why does my mailbox say 15 GB when I pay for Microsoft 365?
Outlook.com gives free accounts 15 GB and lifts active Microsoft 365 Personal/Family subscribers to 100 GB. If you still see 15 GB, the subscription may not be linked to that mailbox, or it has lapsed — check the account the subscription is tied to.
Is the archive mailbox part of my main quota?
No. On business plans the archive is a separate store (starting at 100 GB, expandable to 1.5 TB) and doesn’t count against your primary mailbox limit.
Why won’t my .pst file grow past 50 GB?
The Windows desktop app caps .pst and .ost files at 50 GB by default via the MaxLargeFileSize registry entry. You can raise it in the registry, but large files slow Outlook down — splitting into a second .pst is usually better.
Quick Reference
| Account / mailbox | Storage limit |
|---|---|
| Outlook.com (free) | 15 GB |
| Microsoft 365 Personal / Family | 100 GB |
| Exchange Online Kiosk / Office 365 F3 | 2 GB |
| Exchange Online Plan 1 / Business Basic & Standard / E1 | 50 GB |
| Exchange Online Plan 2 / Microsoft 365 E3 & E5 | 100 GB |
| Archive mailbox (auto-expanding, business) | 100 GB → up to 1.5 TB |
| Desktop .pst / .ost file (default) | 50 GB |
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Related guides: How to clean up your inbox in Outlook · Outlook attachment size limit
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