Steps to changing individual mail user quota

1. Login to the Plesk Control Panel select the domain. 
2. Click on Mail tab and click on the Email ID to change it's quota. 

3. Below are the descriptions of number sequence.

             1. Select the mailbox button to active quota of the mail user.

             2.  Assign quota limit in Specify new size input box.

             3. Select KB (Kilo Byte) or MB (Mega Byte) as unit of the quota limit.

             4. Click OK to confirm the mail user quota.

4. You are done, your mail user quota has been changed or assigned.