Depends on who your SMTP server is.
If you run your own mail server, then it's as easy as getting SMTP to listen on the new port rather than 25. If you do this, though, your SMTP server still has to send out on port 25, since other mail servers won't be listening on any ports other than 25 (or 443, for SSL).
If you don't run your own mail server and use an ISP or similar for mail, then you would have to ask them.