> No, they don't. Most ISPs filter block 25 in both directions, and many block IRC since it's used for botnet coordination. Have you tried it (on an ISP that is commonly used by the technically uninformed majority)?
I'm using the biggest ISP in my country, obviously the commonly used one by the technically uninformed majority. Of course it doesn't filter any ports, doing so would create very bad PR and loss of many customers. Are there really major ISPs filtering? How can such customer-screwing ISPs remain in business outside of a niche-market?