Most M365 mailboxes have a 50 GB or 100 GB quota. Hit the cap and you stop sending — incoming mail keeps arriving until that fills too.

1. Check current usage

In Outlook:

  • File → Info → Mailbox Settings → see total and breakdown by folder
  • Bottom right of Outlook shows percentage used in some configurations

In Outlook Web:

  • Settings → General → Storage

2. Empty the obvious

Quick wins:

  • Empty Deleted Items — right-click the folder → Empty Folder. Doesn't free space until done.
  • Empty Junk Email — same
  • Sent Items — usually the biggest hidden offender; older sent mail can be archived or deleted

3. Find the biggest items

By size:

  • Outlook search bar → Filter → Has Attachments
  • Then sort by Size (column header)
  • Or: Outlook search with size:>20MB (or huge, large, medium)

The 30 PowerPoint decks you sent in 2019 are usually the heaviest. Decide what's still relevant.

4. Save attachments locally, then delete from email

If you need the attachment but not the email:

  • Right-click attachment → Save All Attachments → save somewhere (preferably OneDrive)
  • Then delete the email

5. Archive older mail

The right way to keep space without losing access:

Online Archive (if your account has one)

Some M365 plans include an additional 100 GB Online Archive. Mail moves there automatically by retention policy:

  • Check if you have it: Outlook → look for In-Place Archive or Online Archive as a folder
  • Items there are still searchable from Outlook
  • This doesn't help you free your primary mailbox manually; the retention policy handles it

Local Archive (PST file)

Older approach: move old mail to a PST file stored on your computer:

  • File → New → Outlook Data File
  • Drag old mail into it
  • The PST sits on your hard drive, accessible from Outlook only when added

Downside: PST isn't accessible from web or mobile, and isn't backed up unless your laptop is.

6. Auto-archive

Set Outlook to auto-archive items older than a certain age:

  • File → Options → Advanced → AutoArchive Settings
  • Set frequency and what age to archive
  • Mail moves to a local PST automatically

7. Delete reply chains

When you have a 30-message thread back and forth, you usually only need the most recent one:

  • Open the most recent message — it contains the full quoted history
  • Delete the prior messages in the thread
  • Outlook even has a Clean Up Conversation button (Home tab) that does this automatically

8. Empty calendar / contacts / journal

If you've used Outlook for 10 years, there can be hundreds of cancelled meetings, dead contacts, and journal entries:

  • View → Calendar → switch to List view → delete old items
  • Same for Contacts

9. Convert big attachments to OneDrive links

For ongoing reference attachments, share via OneDrive instead of having multiple copies in mail:

  • Upload to OneDrive
  • Share the link in email
  • Delete the local copies in your mailbox

10. When you're at 100%

If you're at quota and need to free space urgently:

  • Empty Deleted Items first (instant)
  • Sort Sent Items by size, delete a few of the biggest
  • Empty Junk

These three steps usually buy you a few hundred MB to get back under the limit.

When to ticket

Raise a ticket if:

  • You need a quota increase (some companies expand on request)
  • You can't free space because of legal hold
  • An Online Archive should be enabled and isn't
  • You need a backup before clearing space