There is no reason for ISPs to block certain ports, but according to this blog entry, it seems that some still do. SBC Yahoo! DSL blocks outgoing connections on port 25 (SMTP) by default, however, they allow their clients to request the port to be opened. Whether or not SBC Yahoo! DSL unblocks outgoing connections to port 25 for only a particular destination is uncertain. Blocking ports is essentially restricting access to the whole Internet, and I would have sought another ISP if my provider was very strict about accessing third-party SMTP servers or operating servers from my home.
ISPs know which client’s credentials are used to connect to their service. They also have the means of determining which account is abusing their service. ISPs do throttle their clients’ bandwidth already, and they can further diminish the connection quality of abusing, jabbering, or compromised clients. They do not need to block these ports by default. Blocking popular ports, such as HTTP, evidences their lack of enthusiasm to take advantage of the networking tools they have as Internet service providers. It is an indication that the ISP is not strongly motivated to provide unobstructed Internet service to its clients.
I have been running an old computer with OpenBSD as a NAT router since the first day with my ISP. SBC Yahoo! DSL does not support it, but they also do not prevent it. I have also operated a webserver after the first week. Although they have been targetted by lawsuits demanding more protection mechanisms from the Internet, they have been a great ISP. I have been experiencing a lot of dropped connections lately, but I am happy overall for their unobtrusiveness while providing a generally reliable connection to the Internet. I have been an SBC Yahoo! DSL customer for approximately three years.
If an ISP is uncooperative about port unblock requests from hobbyists that are interested in gaining experience in IT administration, then three good solutions are very feasible: sign up for a dedicated hosting plan, experiment with network configuration in a private network, or simply look for another ISP and inform the restrictive ISP on the reason for abandoning them. It is very important that dissatisfied customers discontinue financially supporting restrictive ISPs. There are many ISPs available, and some of them are operated by professional computing hobbyists that simply love what they do and will not prevent their customers from experiencing the pride and joy of running legitimate servers at home.