Port 25/tcp is used for SMTP, the outgoing mail protocol, and is often blocked by ISPs to cut down on spam (whether intentional or due to infection). The block prevents users from sending outgoing mail via any third party mail-hosting services. We've watched what was once somewhat of a scattered practice quickly become an industry standard over the last few years.

The tactic is generally a good idea, and only causes trouble when the ISP fails to clearly inform users of the change, allowing them to employ fairly easy solutions. Some ISPs allow the port to be re-opened upon request, while others - like Comcast - only apply the filters when dealing with spamming accounts in the first place.

Other ISPs take that tactic a bit further, blocking inbound port 25 traffic. Some claim this less common tactic is usually done to prevent users from running a mail server; forcing them to upgrade to a more substantive business account for the privilege. ISPs say this isn't the case; blocking inbound port 25 traffic really helps them keep inadvertent open relays to a minimum, and therefore helps keep their IP blocks off of blacklists.

While we knew ISPs like Canadian cable provider Cogeco were employing this tactic, discussion in our BellSouth forum seems to indicate BellSouth has recently started blocking inbound traffic on port 25. Similar discussion in our Cox forum would indicate that Cox has started doing so as well.

Some argue that measures such as blocking ICMP or in/outbound port 25 TCP traffic eliminates much needed functionality. The general consensus among security experts and spam fighters however is that port 25 blockades (at least outbound) are an effective weapon in the war on spam, and that users can generally find easy workarounds to any inconveniences.

Additional explanation of why ISPs utilize this tactic can be found in this thread in our forums