Greetings,
PHP is required and it is still widely used and supported. ASP is basically the Microsoft equivalent of PHP. Unlike PHP, however, server operators (web hosts) must pay to be able to run ASP scripts. Considering the fee, I'm not sure why you are on an ASP-capable server if no parts of your site are programmed in ASP (maybe I'm assuming wrong here).
It makes sense to develop ListMail on a free language so it can be used on as many servers as possible. Whether your server OS is Windows or Linux (both can run ASP scripts, I believe) it is possible to install PHP from
www.php.net alongside ASP.
I recommend finding a web host and/or manager who understands your needs and the server software.
There are only two web hosts I know of that allow large amounts of opt-in email (up to 25,000 per day). They are mentioned
here.
Please let me know if you have any more questions!
Regards