Summary

This article shows you how to check your Microsoft 365 mailbox storage usage, identify what is consuming space, clean up the biggest offenders, and build a simple routine that prevents quota-related disruptions — before you ever see the "Your mailbox is full" error.

In this guide

• 1. Check Your Mailbox Size and Quota
• 2. Find and Delete Your Largest Emails
• 3. Empty the Deleted Items Folder
• 4. Empty the Junk Email Folder
• 5. Clean Up Sent Items with Large Attachments
• 6. Use the Mailbox Cleanup Tool (Classic Outlook)
• 7. Request an Online Archive Mailbox (When to Ask IT)
• 8. Build a Quarterly Cleanup Habit

Before You Start

• Microsoft 365 mailbox (Exchange Online).
• Access to Outlook on Windows or Outlook on the Web (outlook.office.com).
• No admin rights are required.

Instructions

1. Check Your Mailbox Size and Quota

Determine how close you are to your storage limit before problems start.

In Outlook for Windows (Classic)

1. Open Outlook.
2. Click File in the top-left corner.
3. The Account Information page displays a storage bar under your mailbox name showing how much space is used.
4. For a detailed breakdown, click Tools > Mailbox Cleanup.
5. Click View Mailbox Size….
6. A dialog box shows the total size of your mailbox and a per-folder breakdown. Note which folders are consuming the most space.

In Outlook on the Web

1. Open your browser and go to outlook.office.com. Sign in with your work account.
2. Click the gear icon (⚙️) in the top-right corner, then click View all Outlook settings.
3. Click General > Storage.
4. The page displays your total usage, your quota (typically 50 GB or 100 GB), and a per-folder breakdown.

Verification: If your usage is below 80% of your quota, you have healthy headroom. If you are above 80%, proceed to the methods below to free up space before you get blocked.

2. Find and Delete Your Largest Emails

A few large emails with big attachments often account for a disproportionate amount of mailbox space.

In Outlook for Windows

1. Click in the Search bar at the top of Outlook.
2. Click the Filters option (or the funnel icon), then look for a size filter. Alternatively, in Classic Outlook, switch to the folder you want to search and click Filter Email > Has Attachments to find attachment-heavy emails.
3. To find emails by size manually: click in the search bar, type size:>5MB, and press Enter. This shows all emails larger than 5 MB.
4. Review the results. Look for emails with large attachments that you no longer need.
5. Select the emails you want to remove and press Delete.
6. Repeat with size:>10MB to catch the biggest offenders first.

In Outlook on the Web

1. Click in the Search bar at the top.
2. Type size:large or hasattachment:yes and press Enter.
3. Sort results by size (click the column header if available) to find the largest messages.
4. Select and delete emails you no longer need.

Tip: Before deleting an email with an important attachment, save the attachment to OneDrive or SharePoint first. Right-click the attachment and select Upload to OneDrive (if available) or download it and upload manually. See How to Send Large Files Without Email Attachments for best practices on avoiding large attachments in the future.

3. Empty the Deleted Items Folder

Deleted emails still count toward your mailbox quota until the Deleted Items folder is emptied.

1. In Outlook, scroll down in the left folder panel to find the Deleted Items folder.
2. Right-click Deleted Items and select Empty folder (Classic Outlook) or Empty folder (New Outlook / Web).
3. A confirmation dialog appears. Click OK or Empty to permanently remove all items.

Verification: Your Deleted Items folder shows (0 items) or is empty. Recheck your mailbox size (Method 1) — the freed space appears within a few minutes.

Note: Once emptied, items move to the Recoverable Items folder and remain available for approximately 14 days (depending on your organization's retention policy). After that period, they are permanently deleted.

4. Empty the Junk Email Folder

Spam and junk messages accumulate quietly and consume quota space.

1. In the left folder panel, click the Junk Email folder.
2. Review the contents briefly to ensure nothing was incorrectly marked as junk.
3. Right-click Junk Email and select Empty folder.
4. Click OK to confirm.

Verification: The Junk Email folder shows (0 items). See How to Manage Junk Mail and Spam Filters in Outlook for tips on preventing junk accumulation.

5. Clean Up Sent Items with Large Attachments

Sent Items is often the second-largest folder because every attachment you sent is stored as a copy in your mailbox.

1. Click the Sent Items folder in the left panel.
2. Use the same search technique from Method 2: type size:>5MB in the search bar while inside the Sent Items folder.
3. Review results. For emails where the recipient already received the attachment, deleting the Sent Items copy is safe — it does not affect what the recipient received.
4. Select and delete old sent emails with large attachments.
5. After deleting, empty the Deleted Items folder again (Method 3) to fully reclaim the space.

6. Use the Mailbox Cleanup Tool (Classic Outlook)

Classic Outlook includes a built-in cleanup utility that combines several of the above methods into one place.

1. Click File > Tools > Mailbox Cleanup.
2. Use the options available:
   • View Mailbox Size — see per-folder breakdown.
   • Find items older than — enter a number of days (e.g., 365) and click Find to locate old emails you can archive or delete.
   • Find items larger than — enter a size in kilobytes (e.g., 5000 for 5 MB) and click Find.
   • AutoArchive — click to move old items to a local archive file (.pst). This removes them from your server mailbox quota.
   • Empty Deleted Items — clears the Deleted Items folder.
3. Work through each option to systematically reduce your mailbox size.

Verification: After completing cleanup, click View Mailbox Size again. Your total should have decreased.

7. Request an Online Archive Mailbox (When to Ask IT)

If your mailbox regularly exceeds 80% of its quota despite cleanup, your organization may offer an Online Archive. That is a secondary mailbox with additional storage (typically 50 GB or unlimited, depending on your Microsoft 365 plan).

1. Contact your IT helpdesk and request that an Online Archive mailbox be enabled for your account.
2. Once enabled, an Online Archive folder appears in Outlook's left panel (it may take a few hours to appear).
3. Your organization's archive policy may automatically move emails older than a certain age (e.g., 2 years) to the archive. You can also manually drag emails or folders into the archive.
4. Archived emails are searchable from Outlook and Outlook on the Web — they are not deleted, but moved to a separate storage area that does not count against your primary quota.

Note: Enabling an Online Archive requires admin action. This is one of the few steps in this article that you cannot do yourself. See How to Submit an Effective IT Support Ticket for tips on writing a clear request.

8. Build a Quarterly Cleanup Habit

Preventing "Mailbox Full" errors is easier than reacting to them. A simple habit keeps your storage healthy.

Every 3 months, spend 10 minutes:

1. Check your mailbox size (Method 1).
2. Delete emails with large attachments you no longer need (Method 2).
3. Empty the Deleted Items folder (Method 3).
4. Empty the Junk Email folder (Method 4).
5. Review Sent Items for large attachments (Method 5).

Set a recurring calendar reminder:

1. Open Outlook Calendar.
2. Create a new event titled "Mailbox Cleanup — 10 Minutes".
3. Set it to recur every 3 months.
4. In the notes, paste a link to this article as a reminder of the steps.

Troubleshooting

DANGER

Deleted emails count toward your mailbox quota until you empty the Deleted Items folder. If you delete dozens of large emails and your mailbox size does not change, check whether you also emptied Deleted Items (Method 3).

Symptom / Error	Potential Cause	Solution
"Your mailbox is full" notification	Mailbox quota exceeded	Delete large emails (Method 2), empty Deleted Items (Method 3) and Junk Email (Method 4).
Cannot send but can receive	Mailbox approaching send limit	Microsoft 365 restricts sending before blocking receiving. Free up space immediately using Methods 2–5.
Cannot send or receive	Mailbox fully over quota	Free up space using Methods 2–5 on Outlook on the Web (outlook.office.com) if the desktop app is unresponsive.
Mailbox size unchanged after deleting emails	Deleted Items not emptied	Empty the Deleted Items folder (Method 3). Items there still count toward quota.
"Online Archive" folder not appearing	Archive not enabled or still provisioning	Contact IT to confirm it has been enabled. Allow up to 24 hours for it to appear in Outlook.
AutoArchive moved emails but quota didn't decrease	AutoArchive creates a local .pst, not an Online Archive	Local archive files reduce your server quota. If the .pst was created successfully, the server-side quota should have decreased. Recheck via Method 1.

Related Articles

• How to Fix Outlook Not Receiving Emails (Mailbox Over Size Limit)
• How to Manage Email Attachments and Send Large Files in Outlook
• How to Clean Up Email Conversations in Outlook
• How to Archive Old Emails in Outlook
• How to Manage Your OneDrive and SharePoint Storage Quota
• How to Send Large Files Without Email Attachments
• How to Manage Junk Mail and Spam Filters in Outlook
• How to Submit an Effective IT Support Ticket

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