Port 25

This document is intended to provide you with a brief overview of current email changes being implemented across the internet, and the reasons why these changes are being made. Sonic.net has added a number of protocol options, and services to help you keep up with these ongoing changes, and we hope that this document helps you avoid any problems that may be caused by our, or other providers, implementations of these new email standards.

• How spammers killed port 25
• How does this affect me?
• What can I do about it?
• Does Sonic.net block port 25?

How Spammers killed port 25

According to long-standing norms of Internet use (specifically RFC 821) mail has been transferred using TCP/IP port 25. Mail servers relay messages to each other for delivery to their recipients via this method, as they have for over twenty years. By the very nature of the Internet, there is no master list of which computers are allowed to participate in the email system. This way universities, government agencies, ISPs, and other businesses can freely add mail servers to the network.

As most people that use email are aware, some people caught onto the idea of using email for direct 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, simply taking advantage of their ISP's mail servers. This lead to a great many complaints against the spammers' ISPs, who were forced to protect their businesses by disallowing their customers to send bulk email.

Unable to use respectable ISPs' mail servers to distribute their advertisements, spammers resorted to putting mail server software onto their own computers, to send directly to recipients' mail servers on port 25. Again this resulted in a great many complaints to the spammers' ISPs, and the termination of the spammers' accounts and Internet access.

Some less-than-reputable businesses are willing to provide access to spammers, but it requires a great deal of bandwidth and computer power to send spam continuously. This 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 defensive action to prevent this misuse of their networks. Whenever spam is sent from a zombie system, complaints arrive at the abuse department of the zombie's ISP. This ties up valuable manpower and tarnishes the professional reputations of the ISPs involved. A more efficient, preventative means of controlling this problem is to simply block port 25. Any traffic coming from a customer system that uses port 25 (the mail port) is simply discarded. To make sure that their customers can still send legitimate mail, they allow port 25 traffic which is destined for their own mail servers only.

How does this affect me?

If you use an outgoing mail server that is maintained by somebody other than the company that you connect to the Internet through, you may find that you cannot send mail anymore. In late January, 2005, several large broadband providers started blocking port 25. This resulted in hundreds of customers having to change their mail program settings.

What can I do about it?

If you use one ISP to connect to the Internet, and use another company's outgoing mail server to send your mail, you should use an alternate port. If you are a Sonic.net customer that connects through an ISP that is blocking port 25, you can continue to use mail.sonic.net:

• Outlook Express and Outlook users should use port 465 with SMTP Authentication enabled and SMTPS. Please see our Outlook Express Settings and Outlook 2003 Settings pages for details. Microsoft Entourage for Macintosh also uses this method.
• Users of other mail programs (such as Netscape Messenger or Thunderbird) should use port 587 with SMTP Authentication enabled and SSL or STARTTLS.
• Dial-up users: If you absolutely must be able to send email via a mail server other than Sonic.net's, you may disable the port filtering on a per-call basis by adding ".nofilter" to your login name in your dial-up connection properties.

Does Sonic.net block port 25?

Yes, we block port 25 on all of our dynamic dial-up and by default for all of our DSL customers. For more information please see DSL Firewalling.