Change an Email Account's Quota
This demo will show you how to Change an Email Account's Quota from cPanel. An email quota is the maximum amount of disk space allocated to an email account.
What happens when my mailbox is over quota?
If your mailbox exceeds the quota, there is no room for additional mail and all new messages will bounce back to the sender.
 Click on the "Email Accounts" icon within the "Mail" section of your cPanel
 Locate the email account you would like to modify and click the "Change Quota" link to the right of it
 Enter the amount of disk space you would like to allocate to the email account in the "Mailbox Quota" text field
 Click the "Change Quota" button