SMTP stands for Simple Mail Transfer Protocol – the protocol by which emails are sent. Message data are sent out via SMTP and the traffic is directed into ports for ease of handling.

Think of a port like an old fashioned telephone switchboard, with a operator plugging the incoming call into a jack, to connect it to a specific phone: an SMTP port is designed to direct email through a network, to the recipient.

The original port that was created to deal with SMTP traffic was port 25, and it continues to be used primarily for SMTP relaying: the transmission of email from email server to email server.

It is only needed for use by mail servers.

An SMTP port is the port designated for use by SMTP - this can be ports 25, 465 or 587.

The replacement for ends users to port 25 is 465 or 587.

Port 587 is the default port for SMTP submission on the modern web. It requires that a user authenticate with a username/password combination, and also supports TLS.

Port 465 was originally registered for SMTPS (SMTP over SSL). It was quickly reassigned for a different use and deprecated, but in spite of that many ISPs and cloud hosting providers still support port 465 for SMTP submission.

End users should be using 587 or 465 with SMTP AUTH.

Leaving port 25 open on IP pools that are shared causes your customers to suffer disruptions that are every annoying and can be expensive, especially when 90% of the time the individual affected by the listing is not the person with the infected device.

This policy also pushes the support costs on to the recievers, who must transport and filter the resulting spam.

In some cases it is even worse, since some networks filter against CSS and XBL on their outbound mail servers. The result is that if someone's shared IP is listed because of a proxy, they cannot send email even if they are using port 465/587. This breaks email and is extremely disruptive to conducting business, and makes people really angry.

Limit outbound port 25 to SMPT servers only

Networks should close outbound port 25 for end users, and limit port 25 access to SMTP servers only. This is transparent to end users, who are using port 587 or 465 for authentication.

No end user needs port 25 open for their mobile phone. Only SMTP servers need port 25 open. Routers and firewalls should be shipped with port 25 disabled.

Why should networks do that?

Port 25 is heavily abused and is the spammer's choice port of call.

In the 90s, spammers discovered that port 25 allows people to connect to a mail server and send mail through it, no matter where they were physically located, or whether or not they had any right to access that machine. This "open relay" feature of port 25 was cheerfully abused by criminals until network administrators had had enough, and made some changes.

Port 25 is only needed for use by SMTP servers. If it is left open and accessible, it WILL be abused: spamming is a very lucrative business, and all the more so if the spammers can use other people's infrastructure to send their spam, instead of paying for their own - free "postage"!

In 1998, port 587 was created. Using 587 allows an end user to supply a username and password to the mail server for authentication. If they pass authentication, they can send mail - there is no open relay. By 2005, the open relay issue was essentially solved, and the criminals moved on to other vectors for their spam.

A new version of open relays

Fast forward 20 years, and that new vector is mobile phones.

Despite the recommendations by industry leaders and despite the work that was done to close port 25 on mail servers, many of today's mobile networks have declined to limit port 25 on their IP pools.

On the mobile phone side, a proxy is embedded in (many) apps, the app is installed by an unknowing user, access to the proxy is sold by one or more residential proxy resellers, and the spam is once again flowing freely over open port 25. Of course, the costs of transporting and dealing with all the spam are absorbed by the recievers.

We strongly advise that networks limit port 25 and help clean up the spam.