why does system.net.mail destroy where system.web.mail works

i both system.net.mail system.web.mail work gmail, nonetheless i can't both work smtp.att.yahoo.com.

i smtp settings possess web.config keys. settings work i send controlling system.web.mail, nonetheless destroy system.net.mail.

    <add key="smtpserver" value="smtp.att.yahoo.com"/>
    <add key="smtpserverauthenticateuser" value="ctrager@sbcglobal.net"/>
    <add key="smtpserverport" value="465"/>
    <add key="smtpusessl" value="1"/>
    <add key="smtpserverauthenticatepassword" value="my password"/>

here formula grabs settings, works gmail, fails att.yahoo:

        smtpclient smtp;

        (!string.isnullorempty(util.get_setting("smtpserver", "")))
        {
           smtp = new smtpclient(util.get_setting("smtpserver", ""));
        }
        else
        {
           smtp = new smtpclient();
        }


        (!string.isnullorempty(util.get_setting("smtpserverauthenticatepassword", "")))
           smtp.credentials = new system.net.networkcredential(
               util.get_setting("smtpserverauthenticateuser", ""), 
               util.get_setting("smtpserverauthenticatepassword", ""));

        (!string.isnullorempty(util.get_setting("smtpserverport", "")))
           smtp.port = int.parse(util.get_setting("smtpserverport", ""));

        (util.get_setting("smtpusessl", "0") == "1")
           smtp.enablessl = true;

        smtp.send(message);

is problem?