Unless you can telnet as William said (and I have my doubts there would be a lot more options there anyway), I seriously doubt that you are going to be able to set the port. These home devices are just not made to be very sophisticated or granular. The other problem is that when you have to set a port, it is usually for this reason: The ISP is trying to limit SPAM, and what do they do? They also make you authenticate with a username and password (for example, from within Outlook), and that option is clearly not there.
You also need to realize that you can't just replace that server with anything as you said. Hotmail.com is not going to work since there is no way they relay mail (they send for you, but only directly from their accounts, not relaying from another source). That field is for a server that will replay your mail, and your options are limited there.
You can do one of the following for that field:
- See if your ISP has an e-mail server that will relay for you that authenticates based on subnet (in other words, they see that the originating IP is one of theirs, so they just assume you are OK to relay). Otherwise, you are again in the situation where you don't have the fields necessary to authenticate. That said, most ISP's have stopped offering this service to home customers, but even most business customers still have to provide credentials. I am assuming this is on residential service based on the router.
- Try a proxy service, but I think you will again has the same problems with authentication.
- If you have a server, set up your own mailer, like from within Windows Server 2003 in IIS. This is your best best.
That router was discontinued back in 2008, and developed long before that, when some ISP's still had these relay servers. I think that most home user will now find this type of feature much more difficult to use now, especially if they do not have the option to enter credentials.
My suggestion is to try getting it to work on port 25 first, so you can get the server relay issue addressed first, which is probably an even bigger issue, and then see if you can change the port.