An ISP will block ports -- the goal is top stop spam -- but nothing requires you to use the traditional SMTP ports that are used for spam. Because you're not s

An ISP will block ports -- the goal is top stop spam -- but nothing requires you to use the traditional SMTP ports that are used for spam. Because you're not sending spam. You are sending email to people you know, on a port you have agreed to, who also have reachable IP addresses and will accept connections from your IP address. ISP's are not going to do DPI on open ports like 22, 80, or 443 or any high port, in order to filter out SMTP traffic. Why would they bother? If you must use the existing "email provider" system -- e.g., you need to send commercial email -- then you must use the standardized ports they use, and yes, you must jump through their hoops. And you will experience spam and other nuisances. It's supposed to be open and decnetralized, but the reality is it's quite centralized because everyone relies on "major email providers". But there's nothing about SMTP protocol that requires anyone to use an "email provider" or any specific port, nor to accept email from all IP addresses. Two people can communicate directly via SMTP on a non-standard port and can choose to filter by IP address. Maybe no one does this anymore, but I've done it; it's possible. It's not that difficult to set up. The showstopper is not the set up, nor the ISP filtering port 25. It's that most people do not have a reachable IP address.