Misleading "Disable receiving" feature (patch available)
ISPConfig has a feature to "Disable receiving" for mailboxes. It in fact does not only disable receiving mails by this user, but also sending mails by this user.
A patch is available that splits both functionalities and it should be incorporated into ISPConfig. http://www.howtoforge.com/forums/showpost.php?p=287090&postcount=8