It requires ISPs to block the default SMTP port, Port 25
The South Korean authorities are asking all its internet service providers (ISPs) to block all e-mail sent from anything but "official" e-mail servers in a bid to block spam.
The plan rolled out for this initiative is known as "Block 25". As per this, an ISP blocks the default Simple Mail Transport Protocol (SMTP) port, Port 25, from sending e-mail messages, which will force the users to use their ISP’s mail servers. This, it is hoped, would stop spam.
To be enforced next month, the analysts are not sure if this plan would effectively do that as they feel that spammers could use alternate ports.
James Blessing, a council member of the UK’s Internet Service Providers’ Association, says, "Many corporate mail servers run authenticated access through port 25."
He added, "If you want to connect to that you won’t be able to if you block port 25. You’ll stop people working from home."