As far as I know Samsung isn't a Korean family name, it's just a brand.
That said, are you sure it wasn't the + that caused the problem? I've run into that a few times, presumably when someone tried to roll their own email validation.
It's probably a specific policy of Samsung which doesn't allow the word samsung in recipient addresses. I had the same issue, but with samsung@private-domain.tld
Sometimes you can sign up with the + but when you try to log in either on the homepage or an app, the login is invalid because of that + sign. Different validations. Stopped using that way after getting locked out of accounts 6 months later...
That said, are you sure it wasn't the + that caused the problem? I've run into that a few times, presumably when someone tried to roll their own email validation.