In short, they won't sell your email address or utilize that information for anything other than the operation of the email list.
You're free to unsubscribe from Some Party at any time. You're also free to avoid the whole 3rd party email service by subscribing the Some Party RSS feed and rolling your own. The content of every newsletter will be posted there as well. I'll also be publishing the archives on Medium.
The code for Some Party is published at GitHub and available under an MIT license.
The only third party service used here is Google Tag Manager and Google Analytics. My feelings won't be hurt if you block those on your end, but if you do please say hi once in a while so I know you're out there.