1. The conventional mail arrival notification server. Youbin is a kind of biff in the network age. When youbin is used, the mail spool of a certain, specific machine (mail server) is observed to inform the arrival of mail to a user at an arbitrary machine through the network. On the other hand, the conventional "biff" informs only the user who logs in at the machine with the mail spool. Combining with POP, youbin eliminate a lot of NFS mount of mail spool for mail arrival checking. Since youbin is designed with event driven, the user is promptly informed of status change of mail spool caused by mail arriving and reading. From Debian 3.0r0 APT