Hi,
are you shure you want to send html emails?. The about 30-40% of the internet users who can't or don't want to recieve html-mails will hate you. The rest will not be amused of recieving 10k mails with 100Bytes of information.
But if you really want to do it there is a way of programming sendmail directly. The first line of code to be sent to sendmail is the Text/html header and then the rest of the mail.
regards, alexander