I am marking this as an RFE, but I also view it as a severe bug, since it causes user mail to be deleted without any warning.
Virtualmin currently does not notify users when they are approaching their e-mail (home directory) quotas, not does it notify or bounce messages back to the sender when the quota has been reached. Both of these features are essential to a properly functioning mail system with quotas.
Currently, Virtualmin implements a notification for Bandwidth Monitoring, with options to send e-mail to both the server owner and optionally CC the master administrator (or some other e-mail address). A similar process needs to be created for both user e-mail (home directory) quota notification, as well as Disk Quota Monitoring for server owner notification.
Lastly, the default Postfix configuration with Virtualmin prevents Postfix from bouncing mail or otherwise notifying the sender that their message has not been delivered to the recipient. This needs to be implemented as well.