Safe mail - Security problem

Hello everyone,

I made a contact form in my application with email so that customers can send emails to me from within the application. The code looks like this:

Dim Mail As New MailMessage
            Mail.Subject = "Contact"
            Mail.To.Add(myemail@example.com)
            Mail.From = New MailAddress(myemail@example.com)
            Mail.Body = TextBox4.Text
            Dim SMTP As New SmtpClient("smpt.gmail.com")
            SMTP.EnableSsl = True
            SMTP.Credentials = New System.Net.NetworkCredential(myemail@example.com, mypass)
            SMTP.Port = "587"
            SMTP.Send(Mail)

But the problem is that I've published the application and minutes after that, the email used in the application got hacked. How do I make this secure so that you won't see the password if you would decrypt the .exe? Or that you can't find out the password in another way?