First, I guess your server is on a dynamic IP address. Most dynamic IP addresses are blacklisted by default because they are abused by lots of spammers. There's nothing you can do about it. If you still want to send mails with your server, you should relay through another mail server: http://www.howtoforge.com/forums/sho...ight=relayhost
Also, you should ask your ISP if they block port 25.