Engineering Glossary · Mailbox Ops
Mailbox Quota: The Failure You Hear About by Phone
There is no alert on the sending side and no error on yours; the first signal is a customer asking why their message came back three days ago.
The short answer
A mailbox quota is the storage ceiling on a single email account, and once the box is full the server refuses new mail and returns it to whoever sent it.
What makes it worth engineering around is the direction of the signal. The sender gets a bounce; you get nothing. Nothing in your inbox changes, no error appears in a panel you look at, and the outage can run for days before somebody mentions it on the phone. That makes this a monitoring problem rather than a storage problem — the repair is trivial, and the detection is the part everybody skips.
By the HostingFast team · Reviewed 12 August 2026
100+
Terms in the glossary
2 min
Average read time
Plain
English throughout
24/7
Humans if you get stuck
Quotas stop a single mailbox stuffed with attachments from consuming the whole account's storage. The dangerous part is the failure mode: once it is full, arriving mail is refused, which is invisible from your side and alarming from the sender's.
It is the same short list of offenders every time. Attachments parked in Sent, a Trash folder untouched since 2019, and years of newsletters. That makes a clear-out targeted work rather than an excavation.
Find the storage, do not go hunting for it
The Email Accounts screen in cPanel ranks every mailbox by usage, which identifies the account in one glance. To find the folder inside it, drop to SSH: du -sh ~/mail/yourdomain.com/username/* prints a size per maildir folder and sorts the problem out in about four seconds. The answer is nearly always .Sent or .Trash.
Do not delete by date without looking. Sent is where outbound attachments accumulate, and it is also the only copy of anything a colleague sent from that address. Move it to an archive folder or export it before clearing, then re-run du to confirm the space actually came back.
It is inodes as well as gigabytes
Maildir stores one message per file, so a mailbox holding three hundred thousand small newsletters consumes three hundred thousand inodes while using comparatively little disk. Hosting plans cap file count as well as size, and mail is the usual reason an account reaches the first ceiling long before the second.
Check both. If disk usage looks reasonable but new mail is still being refused, the file count is where to look, and the remedy is different: deleting a handful of enormous attachments frees gigabytes and almost no inodes, while clearing a decade of one-kilobyte newsletters does the opposite.
Set the alert, not the reminder
A calendar reminder to check quotas quarterly is a plan that survives about two quarters. A threshold alert at eighty percent is a plan that keeps working after everybody has forgotten it exists, and the panel exposes per-mailbox usage for exactly this purpose.
Size generously while you are in there. Storage costs a great deal less than a bounced enquiry, so busy boxes belong in gigabytes rather than megabytes. Then review once a year, which is a realistic cadence rather than an aspirational one.
Where this lands on HostingFast
Per-mailbox usage is visible in the panel, SSH is available on the developer-focused plans for the du command above, and mailboxes on your own domain are included with the plan. Support is a human being at any hour, and the scope covers the awkward practical questions other hosts bounce straight back.
Read next: Inode for the other ceiling, Disk Space for the account-wide pool, IMAP for why the mail is server-side in the first place, and Webmail for checking usage without a client.
A hundred terms, each with the mechanism
Every entry here started life as a support ticket. Rather than explain the same term for the two hundredth time, we wrote it down properly — with the command that finds the problem and the threshold worth alerting on.
Support is a human being at any hour, and the scope covers the awkward practical questions other hosts bounce straight back.
- 100+ entries, each with the mechanism spelled out
- Commands you can paste, numbers you can compare against
- Neighbouring terms wired together at the foot of every page
- Written by the engineers who answer the tickets
Why HostingFast
Standard on every plan
Silent failures flagged
Where the error goes to somebody else and not to you, the entry says so and tells you what to monitor instead.
Both ceilings counted
Size and file count are separate limits with different remedies, and mail reaches the second one first.
Commands you can paste
du -sh over the maildir beats an afternoon of clicking through folders looking for the big one.
Alerts over reminders
Anything that depends on somebody remembering is treated as unmonitored, because that is what it is.
Wired to its neighbours
Mailbox Quota runs into Inode, Disk Space and IMAP — the terms that explain where the growth comes from.
This term, landed
Quotas defined, the offenders located by command and an alert threshold chosen — enough to stop the silent bounce.
Quick Start
From order to online
- 1
Rank the mailboxes in the panel
The Email Accounts screen sorts by usage. That names the account in one glance, before anybody opens a mail client.
- 2
Locate the folder over SSH
du -sh ~/mail/yourdomain.com/username/* gives a size per maildir folder. It is Sent or Trash far more often than the Inbox.
- 3
Set an eighty percent alert and size up
A threshold alert survives staff turnover; a quarterly reminder does not. Then give the busy boxes gigabytes rather than megabytes.
Built In
Loaded onto every plan
- Mailboxes on your own domain, included with the plan
- Per-mailbox usage figures visible in the control panel
- Real SSH, plus Git and Composer, on the developer-focused plans
- Human support on duty every hour of every day
- Daily backups with self-service restores you run yourself from the panel
- Upgrades applied in place — changing plan is not a migration
- Per-site PHP version switching from the control panel
- Zero setup charges — there is no joining fee, ever
- Money-back cover: 30 days on hosting plans, 7 on reseller
- Your existing site moved over by our engineers, free of charge
Frequently Asked
What people ask us most often
Where is the space in a full mailbox actually going?
Sent and Trash, in that order, almost every time. Rank the mailboxes on the Email Accounts screen to find the account, then run du -sh over its maildir folders to find the folder. Outbound attachments are the usual culprit, and they are also the only copy of what somebody sent — so archive before deleting, then re-run du to confirm the space came back.
Can a mailbox hit the inode limit before the size limit?
Regularly, and it is the case people miss. Maildir stores one message per file, so three hundred thousand small newsletters consume three hundred thousand inodes while barely troubling the disk figure. If usage looks fine and mail is still being refused, check the file count — and note the remedy is the opposite one, since deleting big attachments frees gigabytes and almost no inodes.
Does email draw on the same disk allowance as the website?
Yes. Disk space is the ceiling on everything the account stores — site files, databases and email all come out of one pool. That is why a mail archive left to grow unattended eventually turns up as a hosting problem rather than a mail one, and why the quota is worth setting deliberately rather than accepting whatever the default was.
Is moving up a tier a migration?
No — upgrades apply in place, from the client area, with no migration and no downtime. The range runs from compact shared accounts through VPS to full dedicated servers, so growth is an account setting rather than a hunt for a new host and a weekend of moving data.
Keep reading
Inode
The file-count ceiling that mail reaches first, and why the remedy is the opposite of freeing gigabytes.
Disk Space
The account-wide pool that site files, databases and email all draw on together.
How to Free Up Mailbox Storage
Clearing the folders that actually hold the space, in an order that does not lose anything.
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A straightforward sequence for a switch your visitors never feel: which files move first, how to shift email across without losing a single message, the right moment to repoint DNS, and the two mistakes behind almost all the downtime we get asked to rescue.
Catch it at eighty percent.
Per-mailbox usage in the panel, SSH for the commands that find it, and mailboxes included with every plan.
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