Question : Telnet to port 25

I am trying to set up a Mozilla Tunnderbird client to connect to my Lotus Notes Domino server via IMAP and SMTP.
IMAP works fine, but I am having problems with SMTP.

So as a test, I went to command line and typed in telnet x.x.x.x 25 and I get a response of "could not open connection to the host on port 25: connect failed" The ISP is AT&T.

If I connect to a PC on a different ISP and try the same command as shown above, I get a 220 response from the server.
So it appears that the first ISP may be blocking the connection?

Could someone explain the process of how this is done and the reasons it is needed?

Thanks

Don

Answer : Telnet to port 25

Here's the AT&T page with info about using port 25 which they normally block: http://help.yahoo.com/l/us/att/smallbusiness/bizmail/pop/pop-12.html  They have a link to a form to request opting out of port 25 filtering.