Softmarket's YMM (Yahoo Mass Messenger) allows you to send the same message to Yahoo users from a list (just a text file pasted into a text box ). You can use multiple accounts in order to avoid yahoo from blocking your account (yahoo blocks an account that sends too many messages per minute to users that are not in their friends.
Freeware download of Yahoo Mass Messenger FREE 1.0.1.121, size
1.51 Mb.