I have lastname@gmail.com, @yahoo.com, @outlook.com, and a couple of others as well as firstnamelastname@gmail.com. I have problems with people signing up with my email addresses constantly. It can be a pain to get this corrected. Some companies refuse to help because the account on their system isn't mine. Other ignore requests.
I had a bank in Australia refuse to do anything because I wasn't the account holder. A quick email to their legal department noting that I was getting personal details in the email and if it didn't stop I'd post the emails to their Facebook page took care of that.
Someone bought a Cooper Mini and used my firstnamelastname@gmail.com email address so I'd get service appointments, notices, etc. Emailing customer service didn't help so I had to email their legal department to get it to stop.
One thing that sometimes help is when there's a link to look at the account I can click that, then click the "Forgot Password" link, make a new password, then log in and change the email address. I'll use their customer support email address or abuse email address.