GMail only allows you to send mail from whoever is logged in. That way you can't send spam from their servers.
You'll have to stand up your own SMTP server or find another server that allows you to send mail on behalf of others.
You can try adding:
mail.ReplyTo = "replytome@gmail.com";
Note: That method is deprecated for the ReplyToList method, use whichever you prefer.
See if they allow that.