You just added a new inbox for a teammate, or one of your existing mailboxes has been quietly collecting newsletters and PDF attachments for a couple of years. Then one morning, mail stops arriving. Not for the whole hosting account — just that one email address — and everything else on the site works fine. That's almost always a mailbox quota problem, not a website problem, and it's easy to fix once you know where cPanel hides the setting.
Symptom: How a Full Mailbox Actually Shows Up
This is different from a full hosting account. Your site loads, FTP works, File Manager uploads are fine — but one specific email address is broken. You'll typically see:
- Senders get a bounce-back with
552 mailbox full,552 5.2.2 Mailbox size limit exceeded, orquota exceededin the delivery report. - The affected user can send mail out fine but stops receiving anything new.
- Webmail (Roundcube/Horde) shows the inbox stuck at the same message count for days, or throws an error when trying to save a draft.
- Outlook or Thunderbird shows sync errors specifically for that one account, while other mailboxes on the same domain sync normally.
- In cPanel → Email Accounts, the usage bar next to that address is at 100% (or very close), while other mailboxes sit well under their limit.
If it's affecting every mailbox and the whole account at once, that's usually the overall hosting disk quota, not this. This post is specifically about the per-mailbox limit.
Cause: Why One Mailbox Fills Up While Others Don't
Every email account you create in cPanel gets its own quota, separate from your total hosting disk quota. You can set it to "Unlimited," but in practice most hosting plans (and a lot of admins, out of habit) cap individual mailboxes at something like 250MB, 500MB, or 1GB when the account is created. That number doesn't automatically grow just because your hosting plan has plenty of free space.
The mailbox itself lives on disk under /home/username/mail/domain.com/mailboxname/, stored as individual message files (Maildir format). A few things drive it toward that ceiling faster than people expect:
- Large attachments. Invoices, screenshots, PDFs — every attachment is stored in full inside the mailbox, and IMAP keeps a copy on the server even after the user "deletes" it if Trash isn't emptied.
- Sent Items and Trash counted against quota. A lot of users don't realize their Sent folder and Trash count toward the same limit as the Inbox. Nobody cleans those out.
- Catch-all addresses. If the domain has a catch-all forwarder pointed at one mailbox, every misspelled address and spam attempt targeting the domain lands in that one inbox.
- An old shared/legacy account that's been in use for years without anyone ever reviewing storage, especially one used as an archive.
- IMAP set up on multiple devices without "delete from server after X days" — every device just keeps everything server-side indefinitely.
Fix: Check and Increase (or Clean Up) the Quota
Start by confirming which mailbox is actually full and by how much.
- Log in to cPanel and go to Email Accounts.
- Find the affected address in the list — the usage column shows a bar like
498 MB / 500 MB. That's your confirmation. - Click Manage (or the three-dot menu) next to that account, then Edit Quota Storage Space.
- Either raise the limit to a more realistic number, or select Unlimited if your hosting plan's overall disk quota has enough headroom to safely absorb it.
- Click Update Email Settings. Mail delivery for that address usually resumes within a few minutes — no need to restart anything.
If you'd rather free up space than raise the ceiling (useful if the account is already near its overall disk quota too), clean the mailbox up first:
- Open Webmail for that address, sort by size or date, and delete large old attachments you don't need.
- Empty Trash and Sent — these count against quota just like the Inbox.
- If the mailbox is used from Outlook/Thunderbird over IMAP, check the client isn't set to keep a permanent local+server copy of everything; archiving locally and removing from the server after a set period stops the buildup from returning.
- For a quick server-side check of actual usage, SSH in (if you have access) and run:
du -sh /home/username/mail/domain.com/mailboxname/That gives you the real number without waiting on cPanel's UI to refresh its usage stats, which can lag by a few minutes after cleanup.
One thing to watch: if a catch-all address is dumping spam into this mailbox, fixing the quota only buys time. Go to Email → Default Address and either disable the catch-all or point it at :fail: so junk mail to bad addresses bounces instead of landing in a real inbox.
Prevention: Stop This From Happening Again
| What to Do | Where | Why It Helps |
|---|---|---|
| Set a sane default quota per mailbox (not Unlimited) when creating accounts | cPanel > Email Accounts | Unlimited hides growth until the whole account quota is at risk |
| Turn on a usage alert | cPanel > Contact Information | Get notified before a mailbox hits 100%, not after mail starts bouncing |
| Set IMAP clients to remove old mail from server periodically | Outlook/Thunderbird account settings | Keeps years of archived mail from silently living server-side forever |
| Review and disable unused catch-all addresses | cPanel > Email > Default Address | Stops spam floods from filling one mailbox |
| Periodically check Email Accounts usage bars | cPanel dashboard | Ten seconds a month catches this before it's urgent |
Mailbox quotas are one of those settings people configure once at account creation and never revisit. A quick quarterly glance at the Email Accounts usage bars in cPanel is usually enough to catch a mailbox creeping toward its limit long before it starts bouncing real customer mail.
Frequently Asked Questions
Will increasing the mailbox quota use up my overall hosting disk space?
Yes — the mailbox quota is a sub-limit within your total account disk quota, not separate storage. Raising it just lets that one mailbox use more of the space you already have. If your account itself is near full, increasing a mailbox quota won't help until you free up overall space too.
Why does the sender get a bounce instead of the message just waiting somewhere?
Once a mailbox is at its limit, the mail server refuses the delivery outright with an SMTP 552 error rather than queuing it, since there's nowhere to store the incoming message. The sender's mail server will usually retry a few times over the next day or two before giving up and returning it as undeliverable.
Can I set mailbox quotas differently for each email address on the same domain?
Yes. Each mailbox created under Email Accounts has its own independent quota field, so you can give a busy support inbox 2GB while leaving a rarely-used forwarding address at 250MB.
Does deleting emails in webmail free up space immediately?
Deleting moves the message to Trash, which still counts against quota until Trash is emptied. Empty the Trash folder explicitly (and Sent, if it's large) to actually reclaim the space.
Is there a way to see which mailbox is closest to full without checking each one manually?
In WHM, under List Accounts or via the account's cPanel > Email Accounts screen, the usage bar is shown per mailbox. For a lot of accounts at once, an SSH one-liner like du -sh /home/*/mail/*/*/ run as root gives a quick sorted view across every mailbox on the server.
