Typically, this is caused by internet service providers blocking the default SMTP port 25, unless the message is being routed through their mail servers. Many of the larger ISPs have begun filtering in an effort to reduce the amount of spam being sent from their networks. To get around this you simply need to configure your mail client to use your ISP's mail server. However, they may require you to do something more specific, so you might want to check with them for further instructions.
Some internet service providers that block port 25 are: AOL, AT&T, BellSouth, CableOne, Cablevision, Comcast, ATTBI, Cox, EarthLink, Flasnet, MediaOne, MindSpring, MSN, NetZero, SBC, Sprynet, Sympatico.ca, Verio, and Verizon.
Please see Article ID ### to tell if your ISP is blocking port 25