Mailbox Full Bounces: Quotas, Causes and How to Fix Them

Short answer: Every mailbox has a storage limit called a quota. When it is full, the server refuses new messages and the sender receives a bounce, often with a code starting 552 or 4.2.2 and a phrase such as “mailbox full” or “over quota”. Free space by deleting large attachments and old folders, or raise the quota, and set alerts at about 80 percent so that you hear about the problem before your customers do.

What a mailbox quota is

A mail server stores each mailbox on disk and gives it a fixed amount of space. The limit may be set by your hosting plan, by the mail provider or by an administrator who wants to keep the server in order. The space counts everything in the account: the inbox, sent items, drafts, archive folders, deleted items that have not been emptied and sometimes the spam folder. With IMAP, which keeps messages on the server, the whole mailbox lives there, so the quota fills over the years. The article on POP3, IMAP and SMTP explains the difference to POP3, where messages are downloaded and removed.

A quota is not a fault. It is a way to stop one account from using all the space. The harm comes when nobody notices that the limit is near.

What the sender sees

When a message arrives for a full mailbox, the receiving server answers with an error. Many servers return a temporary error, with a code beginning with 4, and try again for a few days. Others return a permanent error, beginning with 5, and the sender’s system generates a bounce message at once. Typical texts include “Mailbox full”, “Quota exceeded”, “User is over quota” or the codes 552 5.2.2 and 4.2.2. The article on email bounce codes helps you read them.

The result for you is silent. You see nothing, because the message never reached your mailbox, while the customer sees a failure notice, or worse, assumes you ignored them. Contact forms that send to a full mailbox lose the enquiry completely. Order notifications, password resets and invoices from your own systems can vanish in the same way.

Common causes

Large attachments. A few presentations, scans or video clips can use hundreds of megabytes. Sent items and received copies both count.
Years of mail. Nobody deletes anything, and the archive grows by a little every week.
Automatic mail. Notifications, newsletters, system alerts and reports pile up unread. A forgotten monitoring tool can send hundreds of messages a day.
Spam. Unfiltered spam fills the mailbox, particularly on a catch-all address, see catch-all addresses.
Backscatter and loops. A misconfigured auto-reply can create a stream of replies, as in auto-replies and mail loops.
Shared mailboxes. Several people use one address, and each assumes someone else cleans it. See aliases and shared mailboxes.
A small plan. A cheap hosting plan may allow only a few hundred megabytes per mailbox.

How to fix it now

Check the usage. Log in to the webmail or the control panel of your host and look at the used space of the mailbox, and of each folder.
Sort by size. Most mail programs let you sort messages by size. Delete or save elsewhere the largest ones, especially old attachments.
Empty deleted items and spam. Messages in these folders still count until they are removed permanently.
Save an archive locally, if you need to keep old mail, in a program that can export the mailbox to a file, and store the file with your backups. See backup strategy.
Ask for a higher quota if the mailbox is genuinely in daily use. Your host or provider can raise it, or you can upgrade the plan.
Send a test message from another address and confirm that it arrives.

If senders already received bounces, contact important customers and tell them to resend. Check your contact forms and any systems that send to this mailbox, since their messages may need to be re-created.

How to prevent it

Set a warning threshold. Many hosts and providers can send an alert at, for example, 80 or 90 percent of the quota. Turn it on, and send the alert to a different address.
Plan the size. Give business-critical mailboxes more room than you think they need. Storage is cheap compared with a lost order.
Set retention rules. Decide how long to keep mail, and archive or delete older messages on a schedule. Check legal requirements for invoices and contracts first.
Move large files elsewhere. Share big attachments through links to cloud storage and ask staff to avoid sending them by e-mail.
Filter spam and stop noisy senders. Unsubscribe from newsletters nobody reads and review automatic reports.
Review every quarter. A five-minute look at the usage of each mailbox prevents surprises.

Think about who is responsible. In many small companies, the person who set up the mail years ago has left, and nobody knows where to look when something fails. Write down, in one place, where the mailboxes are hosted, who can change the quota and who receives the alerts. Include the login to the control panel in a password manager that more than one person can reach. When a problem occurs on a Friday evening, a clear note is worth more than any amount of expertise that nobody can find.

It is also useful to distinguish a full mailbox from other limits. Some providers cap the size of a single message, commonly around 20 to 25 megabytes, which is a different error and has nothing to do with total storage. Others limit the number of messages you can send per hour, see sending limits and throttling. When you read a bounce, look at the exact wording and code first, because the right fix depends on which limit was reached.

Special cases

A full mailbox on a shared address needs an owner. Name one person who checks it and cleans it. For a contact address that receives form messages, consider a second route: send the form notification to two addresses, or store the entries in the website database as well, so that a full mailbox cannot swallow an enquiry. The same applies to catch-all addresses, which collect all mail sent to unknown names and fill quickly with spam.

If you changed hosting or mail providers, check the quota on the new side. The default may be smaller than the old one, and a migrated mailbox can arrive already almost full. The checklist for switching email provider includes a step for this. And if the mailbox is not full but mail still bounces, the cause may be different: see order confirmations not arriving for other reasons, such as blocked senders and wrong records.

One more precaution concerns your own outgoing mail. If your sent folder is full of large attachments, you may hit the quota even though few messages arrive. Many programs keep a copy of every sent message by default. Where attachments are common, consider saving sent items to a local archive on a schedule, or storing documents in shared storage and sending links. The change protects the quota, and it also makes it easier to withdraw access to a document later, which is impossible once a file has been mailed to many recipients.

A worked example

A small law office receives enquiries through a form that sends mail to info@ at its own domain. The mailbox has a quota of 1 GB, set by the hosting plan, and has been in use for six years. One Monday a client says that she sent three messages last week and heard nothing. The assistant checks the form and finds no errors, then logs in to the webmail and sees a quota of 100 percent. Over six years, scanned documents and old newsletters have filled it. She sorts by size, saves the 80 largest messages to the office server, empties the deleted items and the spam folder and frees about 600 MB. The host also raises the quota to 5 GB for a small fee and adds an alert at 80 percent, sent to the owner’s personal address. The client resends her messages, and the office adds a rule: attachments over 5 MB go through a file sharing link. No enquiry has been lost since.

Check the rest of your mail setup

A full mailbox is only one reason why mail goes missing. Records and routing matter too. Run a free check with Site AI Audit to see your MX, SPF, DKIM and DMARC results in one report.

The bottom line

A full mailbox silently rejects mail, and the damage is lost enquiries. Check usage, clear the largest items, raise the quota where the mailbox matters and set an alert so that you are the first to know when space runs out.

FAQ

What does “552 mailbox full” mean?

It is a permanent error saying that the recipient’s mailbox has exceeded its storage quota, so the message was refused.

Will the sender try again?

For temporary errors, the sender’s server retries for a few days. For permanent errors it gives up and sends a bounce at once.

Does the spam folder count toward the quota?

On most servers, yes, until it is emptied. The same holds for deleted items.

How much quota do I need?

It depends on the volume of mail and attachments. A busy business mailbox often needs several gigabytes, and a rarely used address much less.

Can I be warned before the mailbox fills?

Many providers and hosting panels offer usage alerts. Enable one at about 80 percent and send it to a different address.