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 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.
Type the following command:
telnet example.com 25
If Port 25 is not blocked, you will get a successful 220 response (text may vary).
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.
telnet: connect to address 188.8.131.52: Connection refused
telnet: Unable to connect to remote host
There is one more tool outPortson 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.
Choose the right port -> https://pepipost.com/blog/25-465-587-2525-choose-the-right-smtp-port/