When a mailbox hits its quota, incoming mail bounces until you free up space. The fastest fix is cPanel's Email Disk Usage tool: it shows exactly which folders are heavy, so you can clear the right thing instead of guessing. Open cPanel from your service's cPanel tile, then look under Email for Email Disk Usage. It lists every mailbox on the domain with a size column, and lets you drill into a mailbox to see its folders.
Most full mailboxes come down to the same few culprits: years of unarchived mail in the Inbox, a Trash or Spam folder nobody empties, or a handful of messages with large attachments. Clear those and you're usually back under quota in a couple of minutes.
Find the heavy folders
Open Email Disk Usage and click into the mailbox that's full. You'll see a per-folder breakdown, typically Inbox, Sent, Trash, Spam, and any custom folders the user created. Sort by size and start with the biggest one. Trash and Spam are worth checking first even if they don't look huge in the list, because most mail clients don't empty them automatically. Messages sitting in Trash still count against your quota until they're permanently deleted, not just moved there.
If Sent is the heavy folder, look for old messages with attachments the sender never cleaned up, a habit of CC'ing large files to yourself adds up fast over a few years.
Bulk-clear old mail
You have two ways to clear space, depending on how surgical you need to be:
- From webmail (Roundcube): log into the mailbox, select Trash or Spam, and use "Empty folder" to permanently delete everything in one action. For the Inbox, sort by date or size, select the oldest or largest messages, and delete in bulk rather than one at a time.
- From cPanel directly: some Email Disk Usage views let you purge a folder's contents without logging into the mailbox at all, useful when you're cleaning up an account you don't have the password for.
If the account holder wants to keep old mail instead of deleting it, archiving to a local client (Apple Mail, Outlook, Thunderbird all support this over IMAP) moves the messages off the server and frees the same space without losing anything.
Confirm the quota cleared
After clearing folders, refresh Email Disk Usage and check the mailbox's new size against its quota. Deletions are immediate, so you should see the number drop right away. If the mailbox is still tight and bounces keep happening, raising the quota is often the simpler long-term fix, see increasing email storage quotas for how much headroom you have against your plan's total disk space.
It's also worth checking whether disk pressure is coming from more than just this one mailbox. If several accounts are large, or your site's files and backups are competing for the same storage, finding what's eating your disk space and inodes walks through the full account-wide picture in cPanel's Disk Usage tool.
If mail already bounced
Anyone who tried to send to the account while it was full got a bounce-back, usually a "quota exceeded" or "mailbox full" message. Clearing the mailbox doesn't retry those automatically, the sender has to resend. If you're troubleshooting the bounce itself, email bounce messages decoded (550, 554, relaying denied) covers what the exact wording means.
When to contact support
If a mailbox keeps filling up right after you clear it, or the Email Disk Usage numbers don't match what you see in webmail, open a ticket from Support → New ticket in the portal. That's usually a sign of a client set to leave mail on the server after download, or a filter forwarding a copy into a folder nobody checks, and it's faster for a real person to spot in the account than to guess from the outside.