A quota is a limit you set. A quota defines the maximum amount of space in megabytes (MB) for any given Zimbra mailbox. All Zimbra mailboxes have quotas. Quotas typically range from 100MB to 11GB, and even greater quotas are practical and allowed by request.

Each mailbox on a Zimbra server is a logically discrete container for someone's email, as well as their documents, calendars, contacts and other information, often collectively referred to as "user data." As you add user data, through your daily receipt of emails, adding calendar items, attaching documents... the logical space taken up by your mailbox (aka Account) grows, as measured by gigabytes (GB). Every mailbox is assigned a maximum number of GB (1024MB) by a Domain Admin.

The maximum amount of space assignable by a Domain Admin in the 01.com hosting environment, without help from 01.com, is 11GB for any one mailbox. When a mailbox reaches its quota, it stops receiving mail, and senders will receive a mailbox full, bounced message. In the 01.com hosting environment, for business accounts, clients may raise their quotas themselves by using their Domain Admin; for personal accounts, clients must request additional quota from 01.com. In Zimbra 5, we have set alerts that send message to clients warning them when they get near their specific quotas.

See Pricing.
Also see: What is a mailbox?