What is Mailbox Quota?
A mailbox quota is a storage limit placed on a user's email account. Once the total size of stored messages exceeds this limit, the server may reject new incoming emails or warn the user.
A mailbox quota is a storage limit applied to an individual email account by a mail server administrator. It caps the total amount of disk space that the mailbox's messages, attachments, and folders may occupy. Quotas are enforced at the storage layer (e.g., on a maildir or mbox file system) or by the mail delivery agent (MDA) and the IMAP or POP3 server.
When a mailbox reaches its quota, the server's behavior is defined by the delivery agent's configuration, typically following RFC 5230 (Sieve mail-filter language) actions or simple reject responses. Common outcomes include the server returning a 552 (mailbox full) SMTP error for incoming messages, bouncing the email back to the sender, or storing the message in a separate overflow area. The mailbox owner may receive a warning (via a Sieve script or an admin notification) when usage approaches the limit.
In a modern stack, quotas are often set per-user or per-domain in the mail transfer agent (MTA) or the IMAP server (e.g., Dovecot's quota plugin). Administrators can configure soft limits (warning at X%) and hard limits (absolute rejection). The quota is enforced regardless of the mailbox protocol: IMAP, POP3, or webmail front ends all see the same storage cap. Exceeding quota does not affect the sending of outgoing mail via the MTA, only the receipt of new mail or the ability to append messages via IMAP copy or POP3 delivery.
Key facts
- Quota limits are set by the system administrator for each user or domain.
- Exceeding quota causes the server to reject incoming mail with a 552 SMTP error.
- Sieve scripts (RFC 5230) can trigger warnings or automatic message rejection at quota limits.
- Quotas are enforced by the MDA or IMAP server; sending mail remains unaffected.
- Soft and hard limits allow warnings before the mailbox is completely full.
How it works in practice
Related terms
References
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