We recommend using Trusted Sender Score (if practical) before buying or signing up to any web site or organization as well as BEFORE submitting your details. That includes any paper forms that request your email address.
If you have already subscribed or purchased from a web site, it's no problem, you can use Trusted Sender Score if you receive an email that seems odd or out of character, especially if the FROM EMAIL ADDRESS contains the same or a similar domain to the one that was you signed up to.