Newtype

 type EmailAddress = EmailAddress.Type

 object EmailAddress extends Newtype[String] { self =>
   def apply(value: String): Option[Type] =
     if (value.contains("@"))
       Some(unsafeCoerce(value))
     else
       None

   // Recommended instance, but not required;
   // use Newtype.Validated to get rid of this boilerplate ;-)
   implicit val builder: HasBuilder.Aux[EmailAddress, String] =
     new HasBuilder[EmailAddress] {
       type Source = String

       def build(v: String): Either[BuildFailure[Type], Type] =
         apply(v) match {
           case Some(r) =>
             Right(r)
           case None =>
             Left(BuildFailure[EmailAddress]("missing @"))
         }
     }
 }