Here is a good list of the SMTP servers for a variety of ISPs.
Adelphia mail.adelphia.net
America Online (AOL) smtp.aol.com
Atlantic Broadband smtp.atlanticbb.net
AT&T (Broadband) mail.attbi.com
AT&T (Dialup) smtp1.attglobal.net
AT&T Worldnet mailhost.worldnet.att.net or imailhost.worldnet.att.net
BCPL mail.bcpl.net
Bellatlantic gtei.bellatlantic.net or smtpout.verizon.net>
Bellatlantic.net smtpout.bellatlantic.net
Bellsouth mail.bellsouth.net
Bestweb smtp.bestweb.net
BEV smtp.bev.net
Blacksburg smtp.blacksburg.net
Blazenet smtp.blazenet.net
Cable One mail.cableone.net
CAIS smtp.cais.net
CAPU smtp.capu.net
Charm.net smtp.charm.net
Charter Communications smtp.charter.net
Citizen’s Internet smtp.swva.net
Comcast smtp.comcast.net
Compuserve smtp.compuserve.com or smtp.site1.csi.com
Concentric.net smtp.concentric.net
Covad smtp.covad.net
Cox West smtp.west.cox.net(west coast users)
Cox Central smtp.central.cox.net (central users)
Cox East smtp.east.cox.net(east coast users)
Cox Business smarthost.coxmail.com
Crosslink smtp.crosslink.net
DCANET smtp-relay.dca.net
Delmarva Online mail-gw.dmv.com
Delta Net smtp.deltanet.com
Direcway smtp.direcway.com
DSL Extreme smtp.dslextreme.com
Earthlink Network mail.earthlink.net or smtp.earthlink.net
Earthlink Network
(International only) ismtp.earthlink.net
Edge.net mail.edge.net
Enter smtp.enter.net
EROLS mail.erols.com
Ezy smtp.ezy.net
Flashcom mail.flashcom.net or smtp.flashcom.net
Frontline.net smtp.fcc.net
Full Channel smtp.fullchannel.net
Gateway.net smtp.Gateway.net
GTI mail.gti.net
HotMail mail.hotmail.com
IBM.Global net smtp1.ibm.net
ioNet Inc mail.ionet.net
Integra smtp.integra.net
Interaccess.com smtp.interaccess.com
Internet America mail.airmail.net
Internet Highway smtp.ihwy.com
ITOL mail.itol.com
Juno smtp.juno.com
Mediacom mail.mchsi.com
Mediaone.net smtp.ce.medione.net
MegaPath mail.megapathdsl.net
MCI mailrelay.mciworldcom.net mailrelay.internetmci.com
Mindspring smtp.mindspring.com
MSN smtp.email.msn.com
Nauticom mail.nauticom.net
Netcom smtp.ix.netcom.com
Netcom (Canada) smtp.netcom.ca
Netscape smtp.isp.netscape.com
NetZero smtp.netzero.net
NYU.edu smtp.nyu.edu
OLG.com mail.olg.com
Omega Communications smtp.i-plus.net
OPTOnline Internet Service mail.optonline.net
Pacbell mail.pacbell.net
Patriot Media smtp.patmedia.net
PeoplePC Online mail.peoplepc.com
Pipeline smtp.pipeline.com
Prodigy smtp.prodigy.net
PSI.net relay.smtp.psi.net
PTD.net promail.ptd.net port 25
QIS mail.qis.net
Qwest Internet Service pop.dnvr.qwest.net
RCN smtp.rcn.com
Rider.edu enigma.rider.edu
RoadRunner smtp-server..rr.com
SBC Global smtp.sbcglobal.net
SBC Global Yahoo smtp.sbcglobal.yahoo.com
Smallville Communications mail.toto.net
SNiP mail.snip.net
Spectrum DSL mail.webstable.com
SprintLink smtp.a001.sprintmail.com
Sprynet m6.sprynet.com
Starpower smtp.starpower.net
Sympatico mailhost.sk.sympatico.ca
UMBC smtp.gl.umbc.edu
USA.NET mail.netaddress.usa.net
US Internet smtp.usit.net
UUNet mail.uu.net
Verizon Internet Services outgoing.verizon.net or smtpout.verizon.net
Wide Open West smtp.mail.wideopenwest.com
XO Communications mail.njd.xo.com or smtp.concentric.net
Yahoo smtp.mail.yahoo.com
Ziplink smtp.ziplink.net