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 inbound port 25 blocking shift from a scattered practice to an industry standard over the last few years.

The tactic is generally a good idea. It only causes trouble when the ISP fails to clearly inform users of the change, allowing them to employ fairly easy workarounds. 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. Comcast started selective blocking of outbound port 25 traffic back in 2004. Some Comcast customers in our forums say it looks like Comcast may now be blocking inbound port 25/tcp traffic in some instances. We've contacted Comcast for confirmation but have not yet received a response.