Best PHP newsletter script?

I'm working with a client on her osCommerce store, and she wants to have several mailing lists for her store. osCommerce has a newsletter function built in but it's pretty rudimentary. What can you recommend that fits these qualifications?

Must be built using PHP and MySQL for easy integration with osC
Handle multiple mailing lists
Double opt-in - the script must send the customer a confirmation email, and the customer replies to the email or clicks a link to confirm their subscription to the list
Send emails out in small batches to as not to trigger spam filters or time out PHP
Schedule mailouts for late at night when traffic and server load are low
Not allow list subscribers to post, only list admin
Send HTML email

I've seen several scripts that claim to do all of this, but I'd rather now waste the time installing things that are a pain to deal with. Anyone have experience they'd like to share?