I want to add a user to an existing site that I have configured in ISPConfig so that he has a webspace account but not an email account, in other words he can access WWW & FTP but doesn't have an email address and can't access Webmail/POP3/SMTP/IMAP. Is this possible?
No, every user in ISPConfig has automatically an email account. But you dont have to use the email account if you dont need it
