SQL Server Insert if not exists

Question

I want to add data to my table, but I only want to add data that is new to my database.

My code:

ALTER PROCEDURE [dbo].[EmailsRecebidosInsert]
  (@_DE nvarchar(50),
   @_ASSUNTO nvarchar(50),
   @_DATA nvarchar(30) )
AS
BEGIN
   INSERT INTO EmailsRecebidos (De, Assunto, Data)
   VALUES (@_DE, @_ASSUNTO, @_DATA)
   WHERE NOT EXISTS ( SELECT * FROM EmailsRecebidos 
                   WHERE De = @_DE
                   AND Assunto = @_ASSUNTO
                   AND Data = @_DATA);
END

The error is:

Msg 156, Level 15, State 1, Procedure EmailsRecebidosInsert, Line 11
Incorrect syntax near the keyword 'WHERE'.

Please can someone help me solve this?