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 list). Also to check if your account(s) is still sending messages you can use a verification account that checks if the messages come through (since yahoo won't notify you that your messages aren't going through).
||Free to try
Windows Server 2008