Port 25 blocking

Firewalls and routers can be programmed to not allow SMTP traffic (TCP port 25) from machines that are not supposed to run Mail Transfer Agents or send e-mail.[3] This practice is somewhat controversial when ISPs block home users, especially if the ISPs do not allow the blocking to be turned off upon request. E-mail can still be sent from these computers to designated smart hosts via port 25 and to other smart hosts via the e-mail submission port 587.