If you need to check disk usage for a mail account, go to Mail tab
and check the Usage gauge for the mail account in question. You can
see actual usage for all mail accounts by clicking Refresh Usage
Stats. Keep in mind that all mail sent to a mail account whose usage
is greater than the mailbox size will be rejected.
Note: Mailbox usage statistics shown under the “Usage” column on the “Email
Addresses” tab may differ from that shown in Statistics or
collected manually via the CLI commands (for example, the -du
utility). Statistics in the “Usage” column reflects only the
Maildir++ size, which does not include service files that can be
stored in the Maildir folders (for example, dovecot.index.cache).