Mailbox quota defines how many MB of mail you can store in your mailbox. You can't store more than allowed by the mailbox quota. It is not included into the site disk space quota. If your hosting plan allows, you can increase the initial free mailbox quota.
To increase mailbox quota:
1. On the maildomain Controls page click the of X MB link in the mailbox Properites entry:
2. Enter new quota size in megabytes and click the Submit button.
From this moment on, you will be charged for the difference between this new amount and the free amount your hosting plan grants. This charge will be added to the recurrent fee at the beginning of each billing period.
You can't change quota for the postmaster mailbox.