A benign explanation is that they probably guessed your email at random the first time and then when you opened one of the emails, you confirmed to the scammers that your email is live and read.
A lot of emails have a 'hidden pixel' in them. Moral of the story - don't open emails from people you don't recognise, no matter how horny you are!
What is a tracking pixel and can I use it to destroy my enemies?
You know how every image on the internet is stored on a server, and your computer automatically downloads them as you browse? Years ago, some genius figured out that your computer’s image requests can let those same servers track your activity across the web — and when it comes to email, they can let the sender see when you’ve opened a given message just by sneaking in an image.
It’s remarkably simple: when you open the email, you automatically download that image, and that image request immediately lets the server know that you’ve opened it.
And, btw, almost every website on the internet will have a Facebook hidden pixel somewhere on the site and that's one of the ways how Facebook knows so much about you.