Port 25 Blocking

Port 25 blocking is a frequent cause of problems in sending email; this page explains the issue and provides solutions.

Issue
TCP port 25 is frequently blocked by Internet Service Providers (ISPs), as an anti-spam technique.

Port 25 is the default port used by SMTP, and blocking it prevents users from connecting to arbitrary email servers (formally, mail transfer agents (MTAs)) and sending email.

To send email on such ISPs, one must either: Port 587 is the official port for sending email on mail submission agents (MSAs), as of RFC 2476 (December 1998), but many clients and servers default to using port 25.
 * use the ISP’s own SMTP server – which may require you to use the ISP’s domain in your email address, rather than your own domain, or
 * connect to your SMTP server at DreamHost on port 587.

Solutions

 * Configure your e-mail client to use port 587, as detailed at those pages, or
 * Use your ISP’s SMTP server – contact them for the address.

Details
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 dialup and broadband customers.

Unfortunately a side-effect of this practice is that it prevents users from accessing our SMTP servers. If you are getting error messages when you try sending mail which state that smtp.domain.com is not responding or not found, contact your ISP, and find out if they're blocking port 25. If they are, you can probably send outgoing mail through their outgoing mail servers, or you can try using port number 587 instead.

Sometimes, however, an ISP will not allow email to be sent through their SMTP server with a different domain. In other words, you may only be able to use their SMTP server if you have your reply address set to your ISP-based email address, not your custom domain name address that you have at DreamHost.

DreamHost does support message submission protocol, as per RFC 2476. This means that you can use port 587 for outgoing mail. If your mailer supports it, you can simply set it to send outgoing mail on this port instead of port 25.

List of ISPs
Some major ISPs that block port 25, and the addresses of their SMTP servers, are:

Fortunately it seems that AOL does NOT do this (as of Feb 2002)!