Hello. Today, I encountered a situation where I had multiple domain names registered. Some were "free floating" without any reference to the site files, but with the hosting provider's registered NS.

However, when I attempted to use one of the domains to send an email, the hosting company refused to register it, claiming that it was already in use on another account. I was shocked to learn that the domain I owned was being redirected to a phishing site. After a brief conversation, the domain was returned to me with a normal stub page. However, I am left wondering who is responsible for this and what actions can be taken to prevent such situations in the future.

If you delegated the domain with the DNS servers yourself, then it's a rhetorical question to ask who is responsible for any issues that may arise.
It's important to delegate domains with DNS servers that you have control over, even if you don't need to create any DNS records for them.

It's unfortunate that we often see websites "started" with enthusiasm (usually on a new domain name), but then quickly abandoned after the first iteration and without paying for web hosting. "Long-term" websites are also abandoned frequently, but there is generally more responsible management of the domain name - with longer registration periods and cancellation of delegation.

For those who wish to maintain an unused domain name, there is the option to park it. You could create a page featuring a logo and a link to a social media page, as I have done with my domain, to maintain its presence.

here are some general suggestions you might consider in such situations.
First and foremost, it is important to ensure the security of your domain name registration account. This includes using strong, unique passwords and enabling two-factor authentication if possible. Regularly monitor your domain names to identify any unauthorized changes or redirects.

If you find that your domain has been redirected to a phishing site, you should immediately notify your hosting provider and domain registrar about the situation. They may be able to assist you in reversing the changes and investigating the issue further. You might also consider contacting the appropriate authorities, such as law enforcement agencies or cybercrime units, depending on the severity of the situation. Taking proactive steps to protect your online presence, such as regularly updating your website's security features and educating yourself about common security threats, can also help prevent similar incidents in the future.

It's recommended to consult with legal professionals who specialize in online security and intellectual property rights for more specific advice tailored to your situation.