How to accept email on port 26, using Iptables port redirection

In some countries, or better said some ISP have started to block port 25 to avoid PC viruses to send copies of themselves by email to all the contacts on you address book.

Well this could be good if you use your ISP smtp server to send email, but what happens if you use your office server, or you are in business trip, and get to a Hotel where you cannot send email through your smtp server because the port 25 is blocked, and you can't use the ISP smtp server because you do not have an account with them.