Knowledgebase: E-mail
Port 25 Blocking
Posted by Support on 20 December 2007 03:36 AM

Port 25 Blocking

Many ISPs are blocking what is called "Port 25" which is the port used to send e-mail. They are doing this to cut down on the amount of spam that is sent from their networks.

All e-mail sent via the Internet is routed through the port 25, the channel used for communication between an e-mail client and an e-mail server. Even though port 25 blocking will probably become an industry standard, however, the filter can create problems for e-mail servers and block legitimate e-mail as well as spam.


Port 25 blocking allows ISPs to block spam sent out through their networks, but it tends to punish the innocent that have a need to send through e-mail servers other than those belonging to their ISP. The ISPs that block port 25 require their SMTP server to be used instead of the remote SMTP server or a SMTP server running on your computer.


You can change in your e-mail program the outgoing mail server port of 25 to port 587 and this should fix the problem.


How the port 25 is used:

All e-mail sent via the Internet is routed through port 25. When an e-mail server that runs on your computer delivers messages, it always uses port 25 to transmit data to remote e-mail servers. Therefore, if your ISP is blocking the port, your messages will not get through.


ISPs that block Port 25

This list contains some of the major ISPs that block port 25 on their servers:

AT&T
BellSouth
CableOne
Charter
Comcast
ATTBI
Cox
EarthLink
Flashnet
MediaOne
MindSpring
MSN
NetZero
PeopePC
SpryNet
Sympatico.ca
Verio
Verizon

If you are an AT&T/SBC Yahoo! DSL customer then you need to enable SMTP Authentication. Within your e-mail program select "Log-On using" for the SMTP/Outgoing mail server settings, and enter your DSL email address and password.


ISP Outgoing SMTP Server Hosts

Below is a list of most common ISP outgoing SMTP Servers. Some ISP's do not allow their users to use a third party mail server to send outgoing mail. In that case you can use your ISP's outgoing SMTP Server. If these addresses don't work, please contact your ISP for their correct SMTP server address.

Adelphia: smtp.blk.adelphia.net
Alltel: mail.alltel.net
AOL: smtp.aol.com
AT&T: mailhost.worldnet.att.net
AT&T Business Internet Service: mail.attbi.com
BCPL: mail.bcpl.net
bellatlantic.net: smtpout.bellatlantic.net
Bestweb: smtp.bestweb.net
BEV.net: smtp.bev.net
Blacksburg.net: smtp.blacksburg.net
BYU: nm.byu.edu
CalWeb: smtp.calweb.com
Charm.net: smtp.charm.net
Charter Communications: smtp.charter.net
chicagonet.net: mail.chicagonet.net
Citizen's Internet: smtp.swva.net
Coastalnet: mail2.coastalnet.com
Columbia: smtp.columbia.edu
Comcast: smtp.comcast.net
Comcast / ATTBI: smtp.comcast.net
CompuServ: smtp.site1.csi.com
CompuServe Classic: mail.compuserve.com
Concentric: smtp.concentric.net
Cox East: smtp.east.cox.net
Cox West: smtp.west.cox.net
Crosslink: smtp.crosslink.net
CU Boulder: eces.colorado.edu
DirecWay: smtp.direcway.com
EarthLink: mail.earthlink.net
Enter: smtp.enter.net
Ezy: smtp.ezy.net
FloydVA.net: mail.floydva.net
ibm.net: smtp1.ibm.net
interaccess.com: smtp.interaccess.com
ITOL: mail.itol.com
Juno: smtp.juno.com
Lycos: email.1stup.com
MCI2000: mailrelay.internetmci.com
McLeodusa: email.mcleodusa.net
mediaone.net: smtp.ce.mediaone.net
Mindspring: smtp.mindspring.com
Mpower Communications: smtp.mpowercom.net
MSN: smtp.email.msn.com
netcom.com: smtp.ix.netcom.com
netexpress.net: mail.netexpress.net
NetHere: mail.nethere.net
Netscape: smtp.isp.netscape.com
Netzero: smtp.netzero.net
NRVi.net: mail.nrvi.net
NTELOS: mail.ntelos.net
NYU.edu: is4.nyu.edu
Omega Communications: smtp.i-plus.net
PacBell: mail.pacbell.net
PeoplePC: mail.peoplepc.com
Prodigy: smtp.prodigy.net
PSINet: bbr0-f1.sna.com
PSI.net: relay.smtp.psi.net
PSU.edu: smtp.psu.edu
PTD.net: mail.ptd.net
RCN.com: smtp.rcn.com
Rider.edu: enigma.rider.edu
Road Runner (Hawaii): smtp-server.hawaii.rr.com
Road Runner (San Diego): mail.san.rr.com
SBC Global Pacbell: smtp.pacbell.yahoo.com
SBC Global Prodigy: smtpauth.prodigy.net
SBC Global SWBell: smtp.swbell.yahoo.com
SBC Global Yahoo!: smtp.sbcglobal.yahoo.com
SDSU: rohan.sdsu.edu
Shentel: smtp.shentel.net
SpeakEasy: mail.speakeasy.net
SprintPCS: smtp.sprintpcs.com
Sysnet: mail.sysnet.net
telocity.com: smtp.telocity.com
UCCS: smtp.uccs.edu
UCSD: smtp.ucsd.edu
ucsf.edu: mail.ucsf.edu
UMBC: smtp.gl.umbc.edu
velocityhsi.com: mail.velocityhsi.com
Verio: mail.verio.net
Verizon: outgoing.verizon.net
Virginia Tech: smtp.vt.edu
Waypoint: mail.wayport.net
Yahoo: smtp.mail.yahoo.com
Ziplink: smtp.ziplink.net
ZNET: mail.znet.com

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