Why is my Outlook mailbox full when it says 15 GB?

Two quotas, not one

A free Outlook.com account has a 15 GB mailbox. It also has 5 GB of Microsoft cloud storage, which Microsoft describes as "shared across your files and photos in OneDrive, attachments in Outlook.com and your Microsoft 365 apps." (Microsoft support)

That second sentence is the whole answer. Message text counts against the 15 GB. Attachments count against the 5 GB. The bar that turns red is almost always the 5 GB one.

Microsoft made the change in 2023: "Since February 1, 2023, your Microsoft cloud storage usage across Microsoft 365 apps and services includes Outlook.com attachments and OneDrive data." (Microsoft support) Before that, a free account could hold 15 GB of anything. After it, the practical ceiling for a mailbox full of PDFs and photos is closer to 5 GB, and that 5 GB is also where your OneDrive files and phone camera roll live.

What "full" does to your mail

The consequence is not a warning banner. Microsoft's words: "You can't send or receive email if you exceed either your cloud storage quota or mailbox storage quota." Incoming messages "are returned to the sender and must be resent." (Microsoft support)

So a mailbox at 4 GB out of 15 can stop receiving mail because of 5 GB of attachments plus a OneDrive folder of holiday photos. The sender gets a bounce. You get nothing, which is the part people notice last.

How to see which quota you are hitting

In Outlook.com, open Settings, then Account, then Storage. Microsoft says the page shows "a breakdown of how much space each folder in your mailbox is taking." (Microsoft support) The cloud storage figure, including attachments, is on the OneDrive storage page or in the Microsoft account dashboard.

If the mailbox number is comfortable and the cloud number is not, you are in the common case: the attachments are the problem, and deleting text-only newsletters will not help much.

Folders that count when you think they do not

• Deleted Items counts "until it is manually emptied or the emails are automatically deleted after 30 days."
• Junk empties itself after 10 days, and counts until then.
• Archive is an ordinary folder in the mailbox. Moving mail there changes nothing about size. Microsoft says so directly for Outlook: using the Archive button "doesn't reduce your mailbox size." (Microsoft support)

Deleting is a two-step action here as everywhere: delete, then empty Deleted Items.

Getting under the limit

1. Empty Deleted Items and Junk. Free and immediate.
2. Find the attachments. In the mailbox, filter or sort by size, or search hasattachments:yes and sort largest first. Save anything you need to your computer, then delete the message. Because attachments are the thing counted against the small quota, this is where the space is.
3. Check OneDrive itself. If the camera roll is backing up to OneDrive, it shares the same 5 GB. Photos may be a bigger share than mail.
4. Use Sweep on the repeat offenders. Sweep can delete everything from a sender, or everything older than ten days from that sender, and keep doing it. It deletes to Deleted Items, so empty that afterwards. (Microsoft support)
5. Clear senders, not messages. Most of a mailbox's volume is a few dozen senders, not a few big files. Deleting a sender's whole history and unsubscribing in one pass is what Inbox Scrub does; it is Gmail-first today, with Outlook on the roadmap.
6. Pay for it. Microsoft 365 Basic is $1.99 a month or $19.99 a year. It gives 100 GB of cloud storage and raises the mailbox to 50 GB. (Microsoft Store) That solves the problem for most people for years, at the cost of paying rent on mail you will never open.

A note on the 15 GB figure

The headline number is not wrong; it is just the less relevant one. If you want a single figure for how much mail a free Outlook.com account really holds, think of it as 5 GB of attachments plus 15 GB of text, with the first limit reached long before the second.