Did you receive a strange letter in the mail or an email from a company claiming that it’s time to renew your domain name? There are companies out there that will 1) try and get your domain transferred to them and 2) try and sign you up for things like an online directory. They will make the letters and emails look like bills or warnings about your domain name. This tactic is often called ‘domain slamming.’
It’s important to know exactly what company your domain is registered with and what company hosts your site. Those are the two legitimate bills you’ll need to pay. All other letters or emails/calls you may receive are likely scams or other companies trying to get you to switch to them.
Making sure private registration is setup on your domains can help hide your contact information from potential scammers.
Wikipedia has a general article about domain name scams that can be helpful.
Namecheap (one of my favorite registrars) also has a great article on their blog about another scam concerning domain name appraisals.
If in doubt, please reach out to me and I will be happy to review everything with you.