The GigaMails can be managed by the Postmaster through the Administration Panel.

To activate the GigaMails that you purchase you simply need to click on the G found next to each account and then, by using the dropdown menu, select the quantity of space that you wish to assign.

You can disconnect the GigaMail at any time simply by clicking on the G again (which will turn grey): in this case you must be careful not to disable the GigaMail service on an email account which already holds over 5GB of mail, and not to set a quantity of space less than the quantity of space that is currently occupied in that account.