Why is my inbox suddenly over its storage limit?

Large attachments, an overstuffed Deleted Items or Junk folder, and old newsletters are usually the biggest space users. Use the mailbox size tool (File > Account Settings, or the admin centre for shared mailboxes) to see what's eating space, then archive old mail to an Online Archive or local PST rather than deleting years of history outright.

Coach's tip: sort by size, not date — a handful of emails with huge attachments is often 80% of the problem.