Why does the Roundcube webmail interface show 0% in the mailbox usage section?

Question

Why does the Roundcube webmail interface show 0% in the mailbox usage section?

Answer

The mailbox in question has it's size set to Unlimited:

Because of this, Roundcube webmail interface usage will show 0%, which basically means infinity:

A quota is needed for the mailbox so that the server can calculate a percentage from a finite amount. Otherwise, the ‘unknown’ will keep showing up in the Roundcube mailbox.
To setup a quota for the email, go to Domains > example.com> Mail > Mail Accounts > Mailbox > General Tab > Set the size > Apply. The quota will be automatically reflected and the "unknown" in the webmail mailbox will be changed to a number from 0 to 100%.