The next thing to do in that case, is to see if there is something on your
computer intercepting the packets before they leave, or if the problem lies
between your cable modem and the other server. Do you have a dial up
account with a different ISP that you could connect to and try to connect to
port 25 of a server? That would tell you for sure if the problem is with
your ISP or software installed on your computer. Likewise, if there are any
other computers connected through the same cable modem in your house that
are able to connect, that would tell you as well.
Barring either of the above, the next step would be to take a network sniff.
If you can show that the transmission is going from your computer to the
cable modem, you will have the ammunition you need to get your ISP to look
into it. If you don't see that, then you'll know its time to look really
hard at software you've installed lately, and possibly run a spyware/ad
removal tool. Ethereal is one popular free tool used for taking network
sniffs, and there are many others, free and not. Just start it up and try
to connect. What you should see is a packet with a source IP of your
computer, and a destination IP of the remote computer/port 25.