When Outlook warns that your mailbox is nearly full or has reached its limit, you lose the ability to send and receive email until you reduce the size. Here is how to clear space quickly and prevent the problem recurring.
Understanding the Warning
Microsoft 365 business accounts typically have a 50GB or 100GB mailbox limit depending on the licence. If you receive a warning, you are approaching that limit. Once the limit is reached, Outlook will refuse to send or receive new messages until storage is freed.
Check your current usage: go to Outlook Web Access (outlook.office.com) → Settings → General → Storage to see exactly how much space each folder is using.
1. Empty the Deleted Items and Junk Folders
These folders often contain gigabytes of accumulated messages that are not actually wanted. Right-click Deleted Items in the folder list and choose Empty Folder. Do the same for Junk Email. This is the fastest way to free large amounts of space immediately.
2. Find and Delete Large Attachments
Large attachments are often the main culprit. To find them:
- In Outlook, go to View → Current View → Manage Views or use the search box
- In the Search box at the top, type:
hasattachments:yesand search All Mailboxes - Click the Size column header to sort by size — delete or archive the largest items first
Alternatively, in Outlook Web Access: use Filter → Sort by → Largest to find big messages quickly.
3. Archive Old Email
Archiving moves old email out of your main mailbox into a separate archive mailbox (for Microsoft 365 accounts) or a local PST file. This reduces your primary mailbox size without deleting anything.
Online Archive (Microsoft 365): If your organisation has enabled the Online Archive, it appears as a separate mailbox in Outlook called “In-Place Archive” or “Online Archive”. You can drag folders into it, or set up auto-archive rules to move items older than a set date automatically.
Auto-archive to PST: Go to File → Options → Advanced → AutoArchive Settings. Enable auto-archive and set it to move items older than 6 or 12 months to a local PST file. The items will be removed from the server mailbox and stored locally.
4. Remove Duplicate Emails
Repeated sync issues or past migrations can leave duplicate messages in your mailbox. Tools like Outlook Duplicate Items Remover (free add-in) or Outlook’s built-in Clean Up feature can help. Go to Home → Clean Up → Clean Up Folder to remove redundant messages in conversation threads, keeping only the latest reply that contains the full thread.
5. Unsubscribe from and Delete Newsletters
If you have years of unread newsletters, select them all and delete in bulk. In OWA, search for a sender name, select all results (click one, then Ctrl+A or use the Select All checkbox), and delete. Repeat for each newsletter sender. Then unsubscribe going forward using the Unsubscribe link at the bottom of each newsletter.
6. Reduce Sent Items Accumulation
The Sent Items folder is often overlooked. If you send large files regularly, this folder grows quickly. Sort Sent Items by size and delete any large attachments you no longer need. You can also go to File → Options → Mail → Save messages and untick Save copies of messages in the Sent Items folder — though most users prefer to keep this enabled for reference.
7. Request a Mailbox Limit Increase
If you consistently need more than your current limit, ask your IT administrator to increase it. Microsoft 365 Business Premium and Enterprise plans allow mailbox sizes up to 100GB, and the Online Archive can provide effectively unlimited storage when enabled. This is the right solution for users who legitimately receive and store large volumes of email.
Preventing the Problem
- Empty Deleted Items and Junk weekly as a habit
- Use OneDrive or SharePoint links instead of email attachments for large files
- Enable auto-archive to move old items out of the primary mailbox automatically
- Check mailbox size monthly in OWA settings so you can act before hitting the limit
