I suggest that the validate function should reside in object EmailValidator. Then, the client merely calls

EmailValidator.validate("foo@foo.com")

It is also worthwhile exploring typealias.

typealias Email = String

Email.validate(): Boolean {

}

The code makes the developer's intention clear: strings which represent emails can be validated.

I believe it is valid to have an extension function to validate a String, when the string represents an email.

Your approach to use a utility object is also a valid approach too.