Further Help - Error Support

Thanks for all the help. After a user enters their data and tries to send it I have this code that does this error check.
If String.IsNullOrEmpty(TextBox4.Text) Then MsgBox("Please Enter A Valid Email Address")
What I want to be able to do is after this box pops up I want the code to stop running and allow the user to enter a email address. Is there any type of code that stops the rest of the code from executing?
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