"Your Apple ID was used to sign in to iMessage on an iPhone 6s"

So I received an email less than an hour ago about my Apple ID was being used to sign in to iMessage on an iPhone 6s. The thing is, I haven't used that email for my Apple ID in a few years because I had to make a completely new account. And so I tried accessing that Apple ID with that email (which is actually my MAIN email I use daily, just not for Apple) and when I went to reset the password, it was going to send it to an email ending with something that doesn't match any of my emails. So, I am not stressed about this or anything but like... is there some way for me to unlink my email from this iPhone because I have no iPhone 6s in my possession and want this person to be able to use the phone for themself without having to use my email. Any help would be much appreciated!!