A full mailbox rejects incoming mail and the sender receives a bounce. Nothing is lost that was already delivered, but everything sent from now on is refused until there is room.

It is rarely the inbox

  • Sent - every message you ever sent, with its attachments. Usually the largest folder by far.
  • Trash - deleted mail is still in the quota until the folder is emptied.
  • Junk - filtered mail still occupies space, often years of it.
  • Archive - which is where "clean up the inbox" usually moved everything.

Find the big ones

Sort by size, not by date. Ten messages with attachments outweigh ten thousand without.

  1. Sort every folder by size — Most clients can; webmail can.
  2. Save the attachments you need elsewhere — Then delete the message, not just the attachment.
  3. Empty Trash and Junk — Deleting to Trash frees nothing.
  4. Compact, if the client offers it — Some formats keep the space until compaction.
Emptying Trash is permanent. Check it for anything deleted by mistake before you do - a full mailbox is urgent, but not that urgent.

Stopping it happening again

  • A retention rule on Junk - thirty days is generous.
  • Attachments in shared storage with a link, not in the message.
  • A quota warning at 80%, so it is a task and not an outage.
If the same mailbox fills every few months, the quota is the wrong size for how it is used. Raising it once is cheaper than clearing it four times a year.