Depending on your needs and situation, you might need to increase or decrease your email quota.
Increasing the email quota will allow user to store more emails before being forced to delete the mails or empty the trash folder.
1) Login to cPanel by accessing the URL http://domainname/cpanel/
2) Click on Email Accounts under “Mail” tab
3) Search for the Email that you wish to change the quota for.
4) Then choose on the selected Email Account -> Change Quota and specify the value that you wish to change and click on Change Quota