DSKTOP CAN’T TELNET TO EMAIL SERVER BUT MAIL CLIENT CAN

Hello:
I Have a Thunderbird Client installed in my destop. It has configured to send email using may email server on port 25, and it works fine: I can send emails .
But if I go to cmd, and try to do:
telnet email.server 25
Could not open connection to the host, on port 25: Connection failed.

My desktop uses XP, and the email server is a RedHat 3.0 Linux.

My PC uses as gateway a router, but in the router I have not restrictions, in fact I log in in my cisco router and I can telnet the email server send test emails with the command sequence EHLO/MAIL FROM/RCPT TO … etc.

Why my PC can connect to email server on port 25 using thunderbird, but not using comand telnet ?

Thanks a lot.