Note: Telia is the name of a large internet service provider (ISP) in Sweden
When I moved recently, my self-hosted email server stopped working. Quick troubleshooting showed that no traffic could neither be sent nor received on port 25, the port used for email. I searched for information about the problem on the internet and found Flashback and SweClockers threads informing me that this was Telia's doing: They block all traffic on port 25 and some additional ports in their network. In my opinion, this is a restriction of online freedom and democracy which today cannot be justified. Communication must be allowed without big brother dropping in.
E-mail is sent and received over network port 25, the "road" the "mailman" takes between mail boxes. Most people's mailboxes are today located at e.g. Google (GMail) or Microsoft (Outlook), but it is also possible to have a private mailbox at home. When Telia and other ISPs block port 25, they are preventing the mailman from traveling home to ordinary people. Instead, everyone is forced to get mailboxes at tech giants like Microsoft or Google, which means they can read all mail which passes through your mailbox (and not just can by the way, but do, all of your mail).
Unfortunately, it is not uncommon for ISPs to block port 25, usually motivated by prevention of spamming. Such collective punishment has never been a good argument. Compare e.g. to a society which outlaws all newspapers and magazines, only because someone is distributing ad leaflets. Today, with SPF able to verify that emails originate from an authorized domain, it is unacceptable to continue blocking port 25. I therefore hope more ISPs follow up on their decisions around firewall policies, something not seeming to happen at Telia, and motivate their standpoints.
The information provided by Telia to their customers on this issue is severely lacking. Only on single line of text on the whole of telia.se contains the following:
The following ports are blocked in Telia's network: 135-139, 445, 25, 53.
Telia's customer support does not seem to have been informed about any firewall, and also finds nothing in their internal documentation about blocked ports. Telia does run a SMTP-relay, but I was only able to find out through various internet forums. When I move or my landlord allows other ISPs, Telia will be quick to go.