500 recipients for 1 message? Wow! I suggest to have a yahoogroups for those. For sure you will forgot some of them.
you can also make a contact list of yahoo, like : asdf @ yahoogroup. com asdf2 @ yahoo. com and by this , you can send several grups at a time.
best way will be configure Gmail in outlook and then send manually 1 by 1; but i think you can't send more than 50 mails from 1 account; for yahoo mail go in support and see for help.
I am thinking if you send out 500 emails at once your easily going to be thrown in spam folders. I tried constant contact and it worked out pretty well.