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)

Features:
* Multiple account send (you can add multiple – unlimited – accounts )
* Easy add recipients (just paste them in the message box) – you can paste even comma delimited yahoo accounts
* Optional use of verification account (so you are sure that your messages are getting through)
* You can use a proxy (socks)
* Configurable wait time between message sends
* Mass add to buddy list (you can choose to add a lot of users at once to your buddy list – skipping the annoying wizzard of yahoo)
* Mass send message to buddies from one account
* Mass send message to ALL buddies in ALL Accounts
* Message sending is throttled in order to avoid Yahoo to block your account from sending