Depending on your situation and needs, you may need to increase or decrease your email quota.
Increasing the email quota will allow users to store more emails before being forced to delete the post or empty the junk folder.
Login to cPanel by accessing the URL http://domainname/cpanel/
Click Email Account under the "Mail" tab
Search for the Email you want to change the quota for .
Change Quota and specify the value you want to change and
Click Update email setting