The quota for an address defines the amount of mail (in Megabytes) that the account can store. When your mailbox exceeds this limit, the system will return any incoming mail to the sender with a message which states that the recipient’s mailbox is full. It is important to keep track of quota usage, because a full quota will not allow you to receive mail.

To change a mail quota, perform the following steps:-

1. Login to Plesk control panel as administrator, https://yourdomain.com:8443
2. Click on the Mail menu.
3. Click on the email address that you want to increase the mailbox size limit.
4. Enter the details about the size limit for the mailbox as stated below.
5. Click OK to save it.