Cpwebhosting.com is now taking a step to help those out there with the port 25 blocking problems that your ISP has done to stop spammers but in the meantime has stopped you from using your Outlook or other programs you are maybe using to send your mail out. We now can help you with that problem, and you can now use our port 25 fix which will allow our customers to use port 26 to send an email.

What is Port 25 Blocking?

Port 25 is a channel used for communication between an e-mail client and an e-mail server, and it is used to send e-mail, many ISP's are blocking this port to cut down the amount of spam that sent from their networks. They are worried about being held liable for their subscribers causing damage to other computer networks by using outside SMTP servers for spamming and developed a set of technical best practices designed to increase the accountability of mail senders while minimizing disruption to mail users.

This industry standard has created the problem for e-mail servers and has blocked legitimate e-mail as well as spam. The ISPs that block port 25 require their SMTP server to be used instead of remote SMTP server.

According to long-standing norms of Internet use (specifically RFC 821) mail has been transferred using TCP/IP port 25.

The Internet allows businesses, ISP's, Universities, governing bodies to freely add mail servers to the network and participate in the email system. This freeness has allowed some people to catch an idea of using the email as a direct mode of advertisement. They started up unsolicited bulk email (spam) systems that would send thousands or millions of copies of a given message at nearly no cost to themselves, just taking advantage of their ISP's mail servers. This lead to a significant number of complaints against the spammers' ISPs, who were forced to protect their businesses by disallowing their customers to send bulk email. Now, these spammers were not allowed to sent emails using respectable ISP's mail servers, so they have put the mail server on their computers to directly send emails to recipients using mail server on port 25. Again this resulted in a significant number of complaints to the spammers' ISPs, and the termination of the spammers' accounts and Internet access.

Some greedy businesses were willing to provide access to spammers, but that requires a significant amount of bandwidth and computing power to send spam continuously. It increases the cost of sending the messages. To mitigate these costs, spammers have resorted to breaking into other users' computers, turning them into “zombie” systems that spew unsolicited bulk email continuously. With the advent of affordable residential broadband connections, a typical home computer can become a prolific source of spam.

Recently, some ISPs, especially those providing residential broadband access, have taken preventive action to prevent this misuse of their networks. Whenever spam sent from a zombie system, complaints arrive at the abuse department of the zombie's ISP. It ties up valuable human resources and tarnishes the professional reputations of the ISPs involved.

A more efficient, preventative means of controlling this problem is to block port 25 only. Any traffic is coming from a customer system that uses port 25 (the mail port) just discards. To make sure that their clients can still send legitimate mail, they allow port 25 traffic which is destined for their mail servers only.

List of ISPs that block port 25 on their servers
Bellsouth Sprynet
AT&T Cox
MindSprint Sympatico.ca
MSN EarthLink
CableOne Verio
NetZero Flashnet
Charter Verizon
People PC MediaOne
Comcast ATTBI

How to detect Port 25 Problem?

  • Port 25 blocked can be seen by running telnet command.
  • In Start/Run box and enter: telnet://[SMTP email server]:25
  • Replace [SMTP emailserver] with the address of any external e-mail server.
  • If the port not blocked, a response starting with the ‘220 ‘ string must be received.

A solution for Port 25 Blocking

  • Cpwebhosting.com now can help you with that problem. You can now use our port 25 fix which will allow our customers to use port 26 to send an email.

The Port 25 blocking in Linux hosting faces some terms i.e. SMTP, Port 25, Port 25 Blocking, in Linux Hosting. These terms explained on following grounds:


SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used for sending and receiving e-mail. However, since it is not much capable to line messages at the receiving end, it is usually used with one of two other protocols, POP3 or IMAP, by which user can save the messages in a server mailbox and download them periodically from the server. In other words, users typically use a program that uses SMTP email server for sending e-mail and either POP3 or IMAP for receiving e-mail. On Unix-based systems, Sendmail is the most widely-used SMTP server for e-mail. SMTP server is most often implemented to operate over Internet port 25.

Port 25:

All e-mail sent via the Internet is routed through the port 25. It acts as the channel used for communication between an e-mail client and an e-mail server. Port 25 is the default port used by the SMTP mail server. Port 25 is used by an e-mail server running on our computer for delivering the messages.

PostCast Server uses the port 25 in two different ways:

* Incoming Connections:

PostCast Server uses port 25 to accept incoming connections from e-mail clients.

* Outgoing Connections

PostCast Server also uses the port 25 for sending. It connects to remote servers and delivers the messages from the Outbox folder.

Port 25 Blocking:

Port 25 blocking is a recurrent reason for hitches in sending an email. Port 25 Blocking prevents users from connecting to random email servers and sending an email. Some Internet providers have recently started blocking port 25 in an attempt to help prevent direct to MX spamming, and abuse of open proxies/relays from their dial-up and broadband customers.

The ill-effect of this practice is that it prevents users from accessing our SMTP mail server. If the error messages are getting at the time of sending mail denotes that smtp.domain.com is not responding or not found in that case we have to contact our ISP and find out if they're blocking port 25. If they are, we can probably send outgoing mail through their outgoing mail servers, or we can try using port number 587 instead.

>> Now we will discuss Port 25 blocking in Linux hosting.

Port 25 is the standard port for SMTP connection. In Linux hosting Message sending is failed due to blockage of this port. We can check the Port 25 blocking in Linux hosting in the following manner:

We can open any of our favorite terminal emulators and execute the following command in order to telnet our website.

telnet my-domain-name.com 25

220-server.hostname.com ESMTP Exim 4.62 #1 Wed, 21 Oct 2009 07:17:30 -0500
220-We do not authorize the use of the system to transport unsolicited,
220-We do not authorize the use of this system to transport unsolicited

If the message states that the connection is refused then an alternative port should be used which is 26 offered by eUKhost.

Following two iptable rule allows incoming SMTP request on port 25 for server IP address (open port 25):

iptables -A INPUT -p tcp -s 0/0 –sport 1024:65535 -d –dport 25 -m state –state NEW,ESTABLISHED -j ACCEPT

iptables -A OUTPUT -p tcp -s –sport 25 -d 0/0 –dport 1024:65535 -m state –state ESTABLISHED -j ACCEPT

In order to block port 25 simply use target REJECT instead of ACCEPT in above rules.

And following two iptables rules allows outgoing SMTP email server request for server IP address

iptables -A OUTPUT -p tcp -s –sport 1024:65535 -d 0/0 –dport 25 -m state –state NEW,ESTABLISHED -j ACCEPT

iptables -A INPUT -p tcp -s 0/0 –sport 25 -d –dport 1024:65535 -m state –state ESTABLISHED -j ACCEPT

Thus, the phenomenon of Port 25 Blocking is similar in every type of web hosting, the difference arises regarding the Operating System used by it. Therefore Port 25 Blocking in Linux Hosting follows the same concept and behavior as, in other hosting services, the change resides in the type of Operating System used in it.