cPanel Mailbox Full: Fix 'Over Quota' Email Bounces

A customer emails you, then calls twenty minutes later asking why you never replied. You did reply - it just bounced back with "mailbox is full" or "quota exceeded," and neither of you noticed. This is one of the most common cPanel email tickets we see, and it's almost never about your overall hosting plan running out of space - it's a limit on one specific inbox.

Symptom: bounces, stuck sending, or mail that never arrives

A full mailbox shows up in a few different ways depending on which side hits the limit:

• Someone emails you and gets an auto-bounce back with a line like 552 5.2.2 mailbox full or Requested mail action aborted: exceeded storage allocation
• You try to send mail yourself and your outgoing message bounces, or your mail client just refuses to send at all
• Webmail (Roundcube, Horde) shows a red "quota exceeded" bar and won't let new mail land in the Inbox
• An IMAP client like Outlook or Apple Mail throws a sync error and stops pulling new messages

Note that this is different from your cPanel account running out of disk space entirely - that failure looks similar but hits every mailbox on the account at once, plus your website uploads and backups. A single full mailbox only affects that one address.

Cause: per-mailbox quota vs. account disk quota

cPanel actually enforces two separate limits, and mixing them up is where most confusion starts:

If a single customer-facing inbox has a small quota (say 250MB from an old default) and years of attachments piling up in Inbox, Sent, and Trash, it'll hit its ceiling long before the account ever does. Trash and Spam count toward the quota too - people forget those aren't automatically emptied.

Fix: find the full mailbox and clear it

Step 1: check usage in cPanel

Log in to cPanel and open Email → Email Accounts. Every mailbox listed shows a usage bar - "412 MB / 500 MB" - right next to it. This is the fastest way to spot the offender without touching SSH. Sort visually if you have a lot of accounts; the ones near 90%+ are your immediate risk.

Step 2: check it from SSH if you need more detail

If you want to see which folder inside a mailbox is actually the problem (Inbox vs. Sent vs. an old newsletter Trash pile), SSH in and look at the maildir directly:

cd ~/mail/yourdomain.com/username
du -sh * | sort -rh

You'll typically see cur, new, and folder names like .Sent, .Trash, .Junk as separate directories - each one is a real IMAP folder and each one counts against the quota. A mailbox that's "only" got 50 emails in the Inbox but 4GB sitting in .Trash is a very common find.

Step 3: raise the quota, or clean it out

Two options, and most tickets end up doing a bit of both:

• Raise the quota - in Email Accounts, click Manage next to the mailbox, change the storage limit, and save. This works as long as the account's overall disk quota has room to absorb it; if it doesn't, you'll just shift the problem up to the account level.
• Clean it out - in webmail, empty Trash and Junk/Spam (most webmail clients don't auto-purge these), then archive or delete old attachments from Sent and Inbox. If the account owner uses Outlook or Apple Mail with IMAP, remind them that deleting locally doesn't free server space until they also empty the client's own Trash folder and it syncs.

What if you can't receive mail at all, even from yourself

If the address is so full it's rejecting everything including your own test emails, you may need to increase the quota first before you can even log in to clean it up - some webmail clients refuse to open a mailbox that's already over its hard limit. Bump the number, log in, delete the bulk offenders, then set the quota back to something sane.

Sending fails too ("552 mailbox full" on your own outgoing mail)

This looks backwards but it happens: some mail setups store a copy of every sent message in the account's own Sent folder before it goes out, so a full mailbox can block outgoing mail as well as incoming. Clearing Sent/Trash usually unblocks sending immediately - no DNS or SPF/DKIM issue involved, despite how alarming the bounce wording sounds.

Prevention: stop this from happening every few months

• Set a realistic default quota. 1-2GB per mailbox is a reasonable floor for a normal work inbox in 2026; "Unlimited" just means the account disk quota becomes the only backstop, and it fills up quietly until everything breaks at once.
• Auto-empty Trash and Junk. In Roundcube, go to Settings → Folders and enable "clear on logout" for Trash and Junk, or set up a Managesieve filter that deletes anything in Trash older than 30 days automatically.
• Archive instead of hoard. If someone needs years of email history, have them archive to a local .pst/.mbox file in their mail client rather than keeping everything live on the server.
• Check quotas quarterly. A five-minute glance at Email Accounts before it turns into a bounced-invoice emergency is cheaper than the support ticket.
• Watch for a pattern, not just one mailbox. If several addresses on the same account are creeping toward full at the same time, that's usually a sign the account's overall disk quota is the real ceiling. Check cPanel home → Disk Usage before you spend time raising individual mailbox limits the account can't actually support.