What is port 25

Port 25, also known as SMTP port (Simple Mail Transfer Protocol) is the default port that mail server use, when installing mail server in companies as an inner email server or as a web mail server the default port that the email server is using is port 25.

Sometimes administrators had to check if port 25 is open, in case when there are problem in sending emails or receiving emails from in or out of the server office location.

Check port 25 using telnet tool:

The quicker and easier tool to use to verify if port 25 is open will be to use the build in telnet tool, in modern operating system the telnet tool is not installed by default and the installation process is within the operation system, an admin is needed to add it to the operation system.

Once you have installed the telnet tool on your operation system you can start the command line interface.

Open the run command (Win+R), type ‘cmd’ in it and press enter.

Then, on the command line (the black screen) you can use the telnet command, type in ‘telnet <server name/ip> 25’ this will try to open a connection form your local host to the server host on port 25.

If the screen become black the port is open if not there may be some option that may cause it, but in basic the port 25 is closed.

Using external tool for verify:

You can also verify that port 25 is open from out side of the company, by using an external tool, like this site, the main site page has an online open port check tool.

This is to verify that users can send email from home, using there own internet connection and not only from the company LAN.

This can be accomplished using telnet from out side the company (like your home computer) or by an online port check tool, like this site.

There are more option to this it that include opening the firewall to enable external connections on port 25.

Some other options that may result of port 25 to be closed can be one of the following:

  • Firewall configuration.
  • Customization in the default configuration due to security settings.
  • Using another secure port for email.
  • Avoiding new viruses to abuse port 25 to spread or send attack using the port.
  • Anti spammers that may use the default port 25 to spread spam emails to companies and home users.
  • And more.

Good To Know:

There are another good to know, free tools that every one can download and use to check the state of port (internal or external).

Putty, this tool has a user interface and is supported more then the telnet protocol, you can use the secure shell (SSH), the rlogin, telnet and more, its free and every one that involved in network have or probably use this tool.

PuTTY a free SSH and Telnet client to check port 25
PuTTY a free SSH and Telnet client (image right for the putty developers at – https://www.chiark.greenend.org.uk/~sgtatham/putty/)

Port 25 is use for SMTP (Simple Mail Transfer Protocol) the basic port for send an receive email, you will need it open but it needs to be monitor to block abuse behavior.

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