Unfortunately, it's not so simply. You must use IP-Authorization to assign a special user account for addresses to blocked. Also, this user must be granted with an access level 0 for object SMTPD. All other addresses need to be assigned by another special account that is granted with an access level 31. Probably, you will need to create a file named noreg by the root Eserv folder and edit Eserv.ini manually to obtain a desired result.