At Mail → Virtual domains you can manage virtual domains, which give a name domain to the mail accounts of the different eBox users.

To create a new domain, you just need to choose a name for the domain and a default size for the eBox users' mailboxes who belong to the domain. Later, you can modify the default mailbox size and force changes at existing mailboxes to any size, from the menu you can find clicking at Edit for the desired domain. If you want to remove one existing domain, just click at the Remove button for that domain.