Is Port 25 blocked?

Port 25, default port of SMTP, is for E-mail transferring. To reduce SPAM, ISPs normally block this port, especially for those household DSL accounts. Therefore, people may think that it is impossible to setup E-mail server at home.
I have experiences of using HKBN and Netvigator, they are not really block all port 25 traffics. They only block outgoing traffics, but not incoming traffic. To setup the E-mail server, we can use the "smarthost" trick.