The Administrator can show users how much space they currently have used of their Mailbox limit. The result can look like

The first step is to set WebMail to use Disk Quota. This option is in Administrator Settings (only a user with administrator permissions can access it via WebMail).

The second step depends on the Complete Icewarp mail server integration option (can be found in Administrator Settings too).
- Complete Icewarp mail server integration is enabled
Mailbox limit is read from Icewarp Config. i.e. each user can have a different mailbox limit and WebMail reads it directly from the Icewarp settings. The Administrator can set this limit in Icewarp Administration Console -> user properties -> Limits tab -> Mailbox size to (kB)

An option Disk Quota Size (MB) in WebMail -> Administrator Settings is hidden and ignored in this case.
Additionally, it is strongly recommended to use Store mail folders in Icewarp mail folder option which is in Administrator Settings as well. Otherwise, the limitation would not work correctly because limits which are set in Icewarp Config are verified according to the Icewarp mail folder not according to the Webmail folder with emails.