A cPanel account's default email address is showing space usage but the email account doesn't contain email.
On cPanel servers the cPanel account email addresses are linked to the cPanel user's default email address so they can all be managed in one place if needed or desired. Dovecot, unfortunately, calculates these linked email accounts space into what is shown as part of the user's quota though. This isn't real usage and is linked usage.
This can be cleared by removing the linked mailboxes.
First, get a list for the user. In this example the user is cptest123 and the domain is cptestacct.com.
doveadm mailbox list -u cptest123
[root@cptestvm ~]# doveadm mailbox list -u cptest123
The two linked mailboxes in this example are
and can be removed by running
doveadm mailbox delete -u cptest123 -s INBOX.acct1@cptestacct_com
doveadm mailbox delete -u cptest123 -s INBOX.acct2@cptestacct_com
followed by running
doveadm quota recalc -u cptest123