Yahoo MassIM - description from Softdepia.com:
Yahoo MassIM is designed to send a message to large amounts of recipients using Yahoo Messenger 5.0. Advanced list management allows you to avoid sending a message to the same user twice, and two Yahoo ID collecting utilities allows you to compile lists of Yahoo IDs either from chatrooms or from Yahoo's Member Directory. The registered version removes the tagline advertisement from outgoing messages and allows you to download a precompiled list of Yahoo IDs.
Yahoo MassIM is a "Free to try" program. You can download and try it for an evaluation period.