A Complete Guide to SMTP Port 25 | Difference Between Ports
Written by
Dibya Sahoo

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A Complete Guide to SMTP Port 25 | Difference Between Ports

Published : November 29, 2019

You may be familiar with the word – PORT. It is a very commonly used word in relation to computers. You might have headered from someone asking if your laptop has a USB or HDMI port. In a layman language, ports are a gateway to connect to an external device for communication. When it comes to Port 25, we are really not talking about these hardware linked port. Port 25 is actually a virtual port coined somewhere near 1982. There was a Request for Comments (RFC) 821 was published, to add port 25 as the default gateway to send and receive emails.

It has been 3 decades and we are still using port 25 as the default port for transmitting email messages. Over the years, few more RFCs came which actually obsoleted the initial SMTP RFC 821. But, still port 25 is there and that also a default port for email transmission.

Over a period of time, developers started facing challenges with default port 25 mostly because of the security concerns, which in turn gave birth to two new ports 587 and 465. Recently you might have started hearing about port 2525, which is nothing but a mirror to port 587. This is primarily used when the port 587 is blocked.

Most Internet service providers now block all outgoing port 25 traffic from their customers as an anti-spam measure. For the same reason, businesses will typically configure their firewalls to only allow outgoing port 25 traffic from their designated mail servers.

Sending emails over port 25 is not really secure and because this is the default port, so it been extensively used by the spammers too.

Basically, when a computer gets infected by a virus it can be hijacked by the virus creators to send out thousands, if not millions of spam emails – the ISPs have to prevent this from happening and blocking the port is the only real solution.

How to check if SMTP Port 25 is blocked on your network?

There are some common error messages which you might encounter;

  • 2014-08-31 17:13:00 SMTP ERROR: Failed to connect to server: Permission denied (13)
  • Connection unexpectedly closed: timed out
  • Timeout::Error (execution expired)
  • S: SMTP ERROR: QUIT command failed: Connection: closed
  • SMTP connect() failed. https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting
  • Mailer Error: SMTP connect() failed.
  • SMTP -> ERROR: Failed to connect to server: Connection timed out (110)
  • SMTP Error: Could not connect to SMTP host.

There can be more. This is just a sample list. All these are errors from different programming environments, but all lead to a common problem that your outgoing port 25 might be blocked.

You should know the error codes well to proactively troubleshoot.

There are multiple ways to check if a particular port is blocked on your network, the simplest one to check this is using the telnet command on your terminal.

    1. Type the following command:

telnet example.com 25

  • View Results:
    If Port 25 is not blocked, you will get a successful 220 response (text may vary).Trying
    Connected to example.com.
    Escape character is '^]'.
    220 cl34.gs01.gridserver.com ESMTP Exim 4.63 Tue, 24 Jun 2008 13:45:04 -0700
  • If Port 25 is blocked, you will get a connection error or no response at all.Trying
    telnet: connect to address Connection refused
    telnet: Unable to connect to remote host

There is one more tool outPorts on Github, which is very user-friendly when it comes to checking the outbound ports. But, yes it has multiple dependencies that need to be installed to start using.

Once, installed just run the below command to check if the port is opened or closed:

outPorts 25

While port 25 is the default TCP port that comes in everyone’s mind to connect SMTP and send emails, but it is definitely not the recommended one. Here is a difference between multiple ports which can help you choose the best TCP port to send emails:

Difference between Port 25 and 465, 587, 2525

Port 25Port 467Port 587Port 2525
Port 25 is to be used for relaying messages between MTAs (Mail Transfer Agents) or from MSAs (Mail Submission Agents) to MTAs.Port 467 is the port chosen for encrypted connections.Port 587 is the one recommended for message submissions by mail clients to mail servers.Port 2525 is it as an alternate port, which mirrors port 587, in the case the other mentioned ports are blocked.
Port 25 is blocked due to the risk of spam and malwarePort 465 should no longer be used at all.The use of port 587 implies the use of authentication, preventing the propagation of spam and malware. It supports TLS encryption.This port also supports TLS encryption.
SMTP was designated to use port 25 in IETF Request For Comments (RFC) 821. IANA still recognizes Port 25 as the standard, default SMTP port.The port is no longer recognized by IANA.This port has secure according to the guidelines set out by the IETF.This port is not endorsed by the IETF nor IANA

I hope this information was helpful to make the right decision on the SMTP port.

SMTP port 25 is the default TCP port for email transmission and Pepipost SMTP relay servers support that too. If you want Pepipost for your SMTP relay and email delivery head over to our website for the best prices.

Related Handpicked Resources

  • Facing trouble in choosing the right SMTP Port?
  • What is SMTP Server?
  • Getting started with Transactional Emails
  • Check if emails from your domain are landing in Spam – Email Blacklist Tool

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Written By: Dibya Sahoo