Email Quota Full: What Happens When a Mailbox Is Full and How to Free Up Space

In short: An email quota is the maximum storage a mailbox may use on the mail server. When the quota is full, incoming messages are not accepted: the sending server either retries for a while or returns the message with a "mailbox full" error. Copies of the messages you send may also fail to save in your Sent folder. To fix it, clear out messages with large attachments, empty the trash and archive the old mail you need to keep.

When a customer tells you "the email I sent you bounced", a full mailbox is one of the first things to check. In this article we explain what an email quota is, what happens behind the scenes when a mailbox fills up, what takes up the space and how to free it safely.

What is an email quota?

On business email accounts, every mailbox has a limit on how much space it may take up on the server. That limit is called the email quota or mailbox quota. It covers every folder stored on the server, not just the Inbox: Sent, Drafts, Trash, Spam and any folders you create. The quota is set when the account is created; for example, you can set the mailbox quota when creating an email account in CWP.

A quota shares the server's storage fairly between accounts and stops a single mailbox from affecting the others. Raising the quota is an option, but the lasting fix is keeping the mailbox tidy.

What happens when a mailbox is full?

When a server tries to deliver a message to you, it learns that your mailbox is full and receives an error code. RFC 5321 defines two replies for this situation:

Bounce messages usually show an extra code such as 4.2.2 or 5.2.2 next to it. RFC 3463 defines this code as "Mailbox full": the mailbox quota has been exceeded and the recipient can delete messages to make room. For temporary errors, RFC 5321 recommends that the sending server keeps the message queued, generally waits at least 30 minutes between attempts and usually keeps trying for at least 4 to 5 days before giving up. Actual timings depend on the sending server's settings.

A full quota does not only affect incoming mail. An email client that uses IMAP may fail to save a copy of the message you sent to the Sent folder on the server, and saving drafts or moving messages between folders can fail too. The IMAP QUOTA extension (RFC 9208) defines the OVERQUOTA code the server returns in that case.

What takes up space in your mailbox?

• Large attachments: Quotes, drawings, photos and scanned documents can take more space than thousands of text-only messages.
• The Sent folder: A copy of every attachment you send is stored here as well.
• Trash and Spam: Deleted messages keep using space on the server until these folders are emptied.
• Years of correspondence: Newsletters, automatic notifications and old project threads add up over time.

With IMAP, messages are stored on the server, so your phone, your computer and webmail all see the same mailbox and share the same quota. We cover this difference in detail in IMAP vs POP3.

How do you free up mailbox space?

1. Sort messages by size: Most email clients and webmail interfaces can sort messages by size. Starting with the largest frees the most space for the least effort.
2. Save the attachment, delete the message: Download the attachments you need to keep to your computer or your company's file storage, then delete the large message.
3. Empty Trash and Spam: Deleting usually just moves a message to Trash. The space is freed once that folder is emptied.
4. Review the Sent folder: Old sent messages with large attachments can take as much space as your inbox.
5. Archive older years: Use your email client's archive feature to move old messages to a local folder or archive file on your computer. A local archive only exists on that computer, so remember to back that file up.
6. Create rules: Rules that move newsletters and automatic notifications into a folder and clean out items older than a set period slow down how quickly the mailbox fills again.

How can you keep your email quota from filling up?

• Size the quota to the job: Give sales or project accounts that exchange many attachments a larger quota than accounts that only receive notifications.
• Share files with links: Sending large files as a share link instead of an attachment protects both your mailbox and the recipient's.
• Set a regular clean-up day: Emptying the trash and checking large messages once a month prevents the problem before it starts.
• Set up devices correctly: Using the same settings on every device stops duplicate messages from piling up. For the steps, see our guide to IMAP and SMTP settings for Outlook, iPhone and Android.

Common mistakes

• Deleting but forgetting the trash: The messages look deleted, but no space is freed.
• Only raising the quota: If habits do not change, the mailbox fills up again after a while.
• A local archive with no backup: If the computer fails, the years you archived can be lost with it.
• Ignoring bounces: A customer who writes to you only notices that the message did not arrive when they read the bounce.

Summary

When an email quota is full, messages sent to you can bounce or be delayed, and copies of the messages you send may not be saved. Clearing large attachments, emptying the trash and moving old mail to a backed-up archive usually solves the problem right away. Sizing the quota of your business email accounts to how they are used and cleaning up regularly keeps it from coming back. For questions about the quota of your mailboxes at ÇAP Hosting, our support team can work through it with you.

Frequently Asked Questions