The default settings for sending email, is to use your Internet connections ISP out bound mail server setting, which is called SMTP.  Change your 'outgoing' SMTP email settings via your email client (Outlook Express, Thunderbird, Foxmail, Eudora, Pegasus Mail etc.) to the correct settings that your ISP supplies you with.

Many of these are listed below:

• Amaze.net.au: mail.amaze.net.au
• Ameritech DSL: mailhost.col.ameritech.net
• AOL: za.mx.aol.com
• AOL: zb.mx.aol.com
• AOL: zc.mx.aol.com
• AT&T: smtp1.attglobal.net
• AT&T: mailhost.worldnet.att.net
• AT&T WorldNet: mailhost.att.net
• AT&T WorldNet: mailhost.worldnet.att.net
• Australink.net: mail.australink.net
• bellatlantic.net: smtpout.bellatlantic.net
• Bluelight.com: smtp.mail.yahoo.com
• BlueYonder: smtp.blueyonder.co.uk
• Bondinet.com: mail1.oznet.net.au
• Breathe: mailhost.breathemail.net
• bright.net: mail.bright.net
• BT Click: mail.btclick.com
• BT Connect: smtp.btconnect.com
• BT Internet: mail.btinternet.com
• BT Openworld: mail.btopenworld.com
• BusinessServe: smtp.businessserve.co.uk
• Cableinet: smtp.blueyonder.co.uk
• Claranet: relay.clara.net
• Comcast: smtp.comcast.net
• CompuServe 2000: smtp.cs.com
• CompuServe Classic: mail.compuserve.com
• CWCom: smtp.ntlworld.com
• DCAnet: postoffice.dca.net
• Demon: post.demon.co.uk
• Earthlink.net: smtp.earthlink.net
• Easynet: smtp.easynet.co.uk
• ee.net: mail.ee.net
• Freeinet.com: mail.ma.freei.net
• Freeserve: smtp.freeserve.co.uk
• Freeserve: smtp.freeserve.net
• Gateway: smtp.gateway.net
• Genie: mail.genie.co.uk
• Greenbank.net.au: mail.greenbank.net.au
• Hotkey.net.au: mail.hotkey.net.au
• ibm.net: smtp1.ibm.net
• IC24: smtp.ic24.net
• icdc.com: mailout.icdc.com
• INFOLINK.com: mail.nfolink.com
• Interlink: mail.your-net.com
• Inweb Networks: post.inweb.co.uk
• IronNet: mail.iron.net
• iwayNET: smtp.iwaynet.net
• Jade Inc.: mail.jadeinc.com
• Lineone: smtp.lineone.net
• Log on America: mail.loa.com
• Logicworld.com.au: mail.logicworld.com.au
• Lycos: smtp.lycos.co.uk
• Madasafish: smtp.madasafish.com
• MegaNet: smtp.meganet.net
• Microsoft's MSN: smtp.email.msn.com
• MidOhio.net: mail.midohio.net
• MindSpring: smtp.mindspring.com
• MindSpring: mail.mindspring.com
• Mistral: smtp.mistral.co.uk
• MSN.com: smtp.email.msn.com
• MSN.DSL: secure.smtp.email.msn.com
• NamesToday: smtp.namestoday.ws
• netcom.com: smtp.ix.netcom.com
• netreach.net: smtp.netreach.net
• Netscapeonline: mailhost.netscapeonline.co.uk
• NetSet: mail.netset.com
• NetWalk.com: mail.netwalk.com
• netzero.net: smtp.netzero.net
• nextek.net: mail.nextek.net
• Nildram: smtp.nildram.co.uk
• nni.com: nni.com
• Northstar Data Systems: mail.nn.net
• NTL World: smtp.ntlworld.com
• OneTel: mail.onetel.net.uk
• On-Ramp: mail.marion.net
• Ozemail.com.au: smtp.ozemail.com.au
• peoplepc: mail.peoplepc.com
• Pipex: smtp.dial.pipex.com
• Prodigy: smtp.prodigy.net
• Prodigy: smtp.prodigy.net
• Purplenet: smtp.purplenet.co.uk
• rcn.com: smtp.rcn.com
• redbird.net: mail.redbird.net
• RichNet: mail.richnet.net
• Road Runner (Insight Communications): smtp-server.insight.rr.com
• Road Runner (Time Warner): smtp-server.columbus.rr.com
• Road Runner (Time Warner): smtp-server.nc.rr.com
• Screaming.Net: smtp.tiscali.co.uk
• Spire: mail.spire.com
  Star Internet: smtp.star.co.uk
• Supanet: smtp.supanet.com
• Telewest: smtp.blueyonder.co.uk
• Telocity: mail.telocity.com
• Tesco: smtp.tesco.net
• Tesconet: mail.tesco.net
• Tiscali: smtp.tiscali.co.uk
• Totalise: mail.totalise.co.uk
• TPG.com.au: mail.tpg.com.au
• TTLC: mail.ttlc.net
• UKGateway: smtp.ukgateway.net
• uu.net: uu.net
• V 21: smtp.v21.co.uk
• Verizon: smtp.verizon.net
• Virgin: smtp.virgin.net
• Vispa: mail.vispa.com
• voicenet.com: mail.voicenet.com
• Waitrose: smtpmail.waitrose.com
• Webcom: smtp.webcom.com
• Which Online: mail.which.net
• Wide Open West: smtp.mail.wowway.com
• Wide Open West (users with @wideopenwest.com): smtp.mail.wideopenwest.com
• Worldonline: smtp.tiscali.co.uk
• Yahoo: smtp.mail.yahoo.com