In order to increase the disk space quota in SmarterMail 4, you must log in to SmarterMail with username "admin" and its associated password.
Once you are logged in, you will need to:
Click on Settings
Click on Domain Defaults
Click on the Limits tab
From this page you can create global default settings that will automatically be used going forward (use 0 for no limit).
Note: Changing these settings has no bearing on domains that have already been setup. In order to apply some or all of the default settings to all domains on your server, you will need to use the Default Domain Settings Propagation option (also found under the Settings menu).