Bell System DSL, Charter Cable, Cox Cable and other ISPs are implementing "port 25 blocking", which means you have to send OUTBOUND email through THEIR server.

Read some of the articles posted on this.

• Article on this issue
• This week's release of the MyDoom virus (and variant) has renewed the debate among many ISP's over the tactic of blocking outgoing port 25 traffic. Port 25/tcp is used for SMTP, the outgoing mail protocol, and is often blocked by ISP's to cut down on spam (whether intentional or due to infection). The block prevents users from sending outgoing mail via any third party mail-hosting services. (read more)
• Some users who upgrade to BellSouth's new Xtreme 3Mbps tier are noticing the company has taken the opportunity to block their port 25 outgoing traffic. (read more)
• Comcast, the nation's largest broadband Internet service, this week began selectively blocking a network loophole commonly exploited by spammers. (read more)