public class MailManagerTest
{
  [Fact]
  public void NotValidEmail()
  {
      //Arrange
    var mailManager = new MailManager();
    const string mailAddress = "john.smith.company.com";

      //Act
      bool isValid = mailManager.IsValidAddress(mailAddress);

      //Assert
    Assert.False(isValid, $"The email {mailAddress} is valid, but it shouldn’t");
  }
}