Author of Kill the Newsletter! here. First, thank you for interest, I’m glad the tool was useful to you. While of course Kill the Newsletter! does work as a throwaway email address, I ask you to please not use it for that. I fear this might result in @kill-the-newsletter.com email addresses ending up in blacklists. Or that the service grows in a way that I couldn’t support with my $0 budget :) Finally, I can’t make strong guarantees of privacy and security for incoming emails. The security is through obscurity of the generated email address and corresponding feed URL, which is not a strong model. This is generally fine for newsletters—whose contents are public anyway—but not so much for signing up for a private Twitter account.
Also, there are other tools like https://www.mailinator.com/ designed specifically for this use case. You’re much better off with them!
And, when you want to convert that newsletter into an RSS feed, I hope you consider Kill the Newsletter! an option :)