Oh now THAT'S interesting, at least from a security perspective.

Just tried to register a second nexon account on my mule accounts email address, it asked me to enter my email, I did, and then it gave me this message.

 Spoiler

Verify Your Email

To complete your registration, check your REDACTED inbox for an email from verify@nexon.net.

And I checked my email account.

 Spoiler

Greetings,

Another user has tried to register an account with the email address REDACTED. For your account security, we recommend you update your password and secondary security.

Visit the Account Settings page at anytime to update your information.

— The Nexon Team

Now that's neat.
They give an all clear on the registration end, but then give a warning to the actual email account.
Kinda interesting of them to have that little bit of misdirection.