You can't block port 25:

25 TCP Simple Mail Transfer Protocol (SMTP)—used for e-mail routing between
mail servers

I am not sure you can use a dynamic IP, I think you need a static IP. Is this
SMTP server going to be on an intranet or the internet? If you need to SMTP to
servers on the Internet, that is going to be quite a chore. Are you just
experimenting or is this a real server?