With 20i all mailboxes have 10 GB of data storage. It's unlikely that you should ever fill this unless you're receiving a lot of spam, or have a lot of emails with large attachments. All the emails and attachments you have in your mailbox are stored on a server, and each mailbox is given a specified amount of space on the server, 10 GB in the case of 20i. Once that space has been filled by email and data, you will no longer be able to receive emails.