Dutch internet services provider Ziggo will block outgoing traffic through port 25. This means that all Ziggo-subscribers can only use the Ziggo server for their email. Ziggo sees this decision as a necessary measure against the sending of unsolicited bulk emails.

Transmission Control Protocol (TMC) port 25 is the channel which is used for SMTP data. SMTP means Simple Mail Transfer Protocol and is basically the standard for sending email over the Internet. Gradus Vos, a spokesman for Ziggo, confirms that it is no longer possible to send mail through other SMTP servers than that of Ziggo, unless another TMC port is used.

Some Ziggo customers have an alternative SMTP server, but this option will expire soon. Other ISPs in The Netherlands, such as KPN, have taken similar decisions in the past. According Mr. Vos, Port 25 is often blocked to prevent users from sending spam messages. Sometimes, customers are unaware that they clutter the web with spam, for example because they are part of a botnet.

Very few Ziggo-subscribers are expected to actually notice the new policy of their ISP, as the average customer does not engage in setting up their own email servers. Mr. Vos advises subscribers who still want to use a customised mail server to create this through a virtual private network. "The alternative for this decision would be allowing that our services are used to send tonnes of spam messages and that we will appear on all kinds of blacklists," said Fox. Port 25 is often blocked to prevent users from sending spam messages, for example because they are part of a botnet.

Source: Tweakers.net 

(References here and here)