You have to set up mail forwarding from off-site, i.e. from outside your ISP. There's no way to tell all hosts to send e-mail to you on a different port. The standard is port 25/TCP and there is no provision to change that.
What you can do is direct your e-mail to a different site that is outside your ISP, and have that site send e-mail to you on a port other than 25/TCP. At one time
www.dyndns.org was offering such a service, but I don't know if it still exists and I don't know what the quality is like.
The easiest way to receive SMTP on a different port is to have your firewall listen on a different port and just redirect that to port 25 on your e-mail server. That way you don't have to mess with your e-mail server configuration, which can be very messy considering that a number of programs may try to send e-mail on port 25, and that may be hard-coded into the application.