smtp Port 25 issue

I am running a Panther server (10.3.9) on a Xserve which provides email services for my organization. Lately, a number of users have been complaining that they are unable to send mail from their home networks. However, email works fine when sent within the office network. It also works for some users from their homes. I am suspecting that the problem is because of ISP providers blocking port 25. Here is my question: How do I get around this without using the ISP smtp servers. Almost everybody has a different ISP and getting their smtp details and configuring them for every user is a hassle. Is it possible to make the Panther server listen on port 587 also (the keyword here is "also"). Would I have to setup any forwarding in my firewall? Thanks in advance, J