Quick answer
Empty Deleted Items and Junk, purge Recoverable Items, find and remove large-attachment emails, and turn on your Online Archive if your licence includes it. Check your usage in Outlook on the web.
WHY IT HAPPENS
You've simply hit the ceiling.
Microsoft 365 mailboxes have a size limit — commonly 50–100 GB depending on the licence. As you near it, Outlook warns you; if it fills completely, you stop being able to send or receive. Years of large attachments, a bulging Sent Items and a Deleted Items folder that never truly empties all add up faster than most people expect.
THE FIX
Work through these in order.
- Empty Deleted Items and Junk EmailRight-click each folder and choose Empty Folder. This is the quickest space back — deleted mail still counts against your quota until the folder is emptied.
- Find and clear big emailsSort your Inbox by Size, or search hasattachments:yes, to surface the largest messages. Save anything worth keeping to OneDrive or a file share, then delete the email.
- Purge Recoverable ItemsIn Outlook on the web, open Deleted Items → "Recover items deleted from this folder" → purge. This hidden retained space counts too and is often the surprise culprit.
- Check your actual usageIn Outlook on the web go to Settings → General → Storage to see exactly how much you're using and which folders are largest.
- Turn on the Online ArchiveIf your licence includes it, use the Online Archive — it moves older mail out of your main quota into a separate archive you can still search.
- Set it to self-manageConfigure AutoArchive or ask IT to apply a retention policy so old mail is archived automatically and you don't have to do this again by hand.
- Don't forget Sent ItemsLarge Sent Items is a common blind spot — sort it by Size and clear out big outgoing attachments too.
- Ask IT about your planIf you're on a small plan, check with IT whether your licence includes a larger mailbox or an archive that just needs switching on.
If "mailbox full" keeps coming back — it usually means an archive isn't switched on, or old mail should be living in one. That's a quick IT fix that sets your mailbox to manage itself and stops the warning returning every few months.
