Unfortunately, the vast majority of ISPs in the States and Canada block inbound port 25 from anywhere, as well as outbound port 25 to anything other than the IS

Unfortunately, the vast majority of ISPs in the States and Canada block inbound port 25 from anywhere, as well as outbound port 25 to anything other than the ISP-provided mail servers (to say nothing of spam blacklists having ISP-numbered pools listed). That means that the majority of users are going to have to go with a hosted solution or roll their own using a server (or VPS) that's in a datacenter somewhere.