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SMTP Issues
Posted by SITEWORKS SYS on 04 January 2006 05:13 AM

SMTP / Port 25 blocking by ISPs

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.

There is a detailed article on this subject that can be read here.... at the bottom of the article is a partial listing of ISP's that are currently blocking sending email in this fashion:

Here are some ISPs that block Port 25:

AT&T
ATTBI
BellSouth
CableOne
Charter
Comcast
Cox
EarthLink
Flashnet
MediaOne
MindSpring
MSN
NetZero
People PC
SBC / Yahoo
Sprynet
Sympatico.ca
Verio
Verizon

Firsthand knowledge shows us that sometimes an ISP allows sending and then later in the same day it does not. This seems to suggest that the block may be triggered by volume. As more and more ISPs prevent SMTP relay to domain names off-site, it is preferable to always use your ISP SMTP address in your email client programs SMTP set up. You can still use you@yourdomain.com in the FROM and REPLY TO sections of the email client program set up.

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