If you look at the documentation, you'll see there's two overloads. One takes a MailMessage parameter, the other takes four strings. If you provide any other combination of parameters, VB .NET doesn't have a method to call and will complain.

You're trying to use the one that takes four strings. You're providing five strings and an empty argument. That's not going to work. stanav is trying to tell you this. There can't be something missing in your blank space because there is no Send() that takes five parameters. stanav's suggestion is correct.