Hi,

I use phplist which is opensource, which is very easy to use and delivers mails to around 18000 users every week. I do not know whether your email server is customizable, but yahoo does check how many mails came from a certain IP address and to how many of its users in a given timeframe. Try making the time-gap between two emails 3 seconds if your number of users is not very large. This is possible in phplist. I dont know about commercial applications. This worked for me. Guess you could give it a try.