Fix Disk Quota Exceeded Mailbox Full Errors
If you or others get an error when sending mail to your email account, you may need to increase the email account's disk quota.Log in
to cPanel and check that the Disk Space is not full. If it is, ignore the rest of this guide. Instead, you will need to either upgrade your account by submitting a ticket through or find and clear disk space on your account.
Click on the Email Accounts
icon in cPanelFind
the account who's Quota is full and click Quota
Increase the Quota and hit save