Telnet port 25 / Telnet SMTP commands / Check SMTP Port  |  17-Feb-2025

SMTP check port 25 with the Telnet command

You can check any SMTP Server on SMTP port 25 with the following Telnet commands. Open a command line and type: 

telnet smtp-server.domain.com 25

This can also be achieved with the free PuTTY software. Check the "Telnet" radio button and type 25 instead of 23 in the "Port" textbox. In newer PuTTY versions, select "Other" and choose Telnet.

If your server is online a connection will be established on port 25 (SMTP). A Microsoft Exchange Server answers with the following output:

220 mailserver.domain.com Microsoft ESMTP MAIL Service ready at Mon, 16 Dec 2024 20:20:59 +0100

When you type the ‘help’ command the available commands are listed:

214-This server supports the following commands:
214 HELO EHLO STARTTLS RCPT DATA RSET MAIL QUIT HELP AUTH TURN ATRN ETRN BDAT VRFY

 

Sending Emails from the command line

Do the following to send an Email from a command prompt. You should not make any mistakes when typing. The backspace key does not work in this mode:

helo exchange.domain.com # if nothing happens, repeat this
250 exchange.domain.com Hello [10.1.7.105]

mail from:<myname@domain.com>
250 2.1.0 myname@domain.com....Sender OK

rcpt to:<recipientname@domain.com>
250 2.1.5 Recipient OK

data
354 Start mail input; end with <CRLF>.<CRLF>

subject: This is a test mail
to: recipientname@domain.com
This is the text of my test mail.
.
250 2.6.0 <exchange.domain.com> Queued mail for delivery

quit

In newer Exchange versions the brackets are obsolete. If the mailserver returns a "syntax error" after the "mail from:" command, you've probably forgotten to put the mail address in brackets <>.

The better way to check SMTP port 25 (free command line tool, no installation required)

You can check TCP port 25 (and other ports) with the TCP port check command line tool (www.portcheck-tool.com). It can be used in a similar way to the ping command. Just specify host and TCP port:

PortCheck.exe MyServer 25

or continuous

PortCheck.exe -t MyServer 25

If the server is responding, the line is displayed in green, otherwise in red. The result looks like this:

C:\Users\admin>portcheck -t smtp.office365.com 25 smtp.office365.com - Port 25 - OPEN - 30ms smtp.office365.com - Port 25 - OPEN - 34ms smtp.office365.com - Port 25 - OPEN - 40ms smtp.office365.com - Port 25 - OPEN - 47ms smtp.office365.com - Port 25 - OPEN - 30ms smtp.office365.com - Port 25 - closed smtp.office365.com - Port 25 - closed smtp.office365.com - Port 25 - closed smtp.office365.com - Port 25 - closed smtp.office365.com - Port 25 - OPEN - 30ms smtp.office365.com - Port 25 - OPEN - 29ms smtp.office365.com - Port 25 - OPEN - 41ms smtp.office365.com - Port 25 - OPEN - 33ms smtp.office365.com - Port 25 - OPEN - 38ms smtp.office365.com - Port 25 - OPEN - 32ms smtp.office365.com - Port 25 - OPEN - 34ms smtp.office365.com - Port 25 - OPEN - 30ms

PortCheck offers many additional options like sending e-mails in the case of a failure or notifying with a beep. PortCheck can check multiple hosts on multiple TCP ports with a single command. The PortCheck tool is also part of the PingTool software, which is freeware, too.

...or use PingTool - the graphical Port Check Tool

Ping Tool is more than just another ping tool for Windows 11: It's also a graphical Port Check Tool an a Wake on Lan tool for IT professionals. It's a portable tool and it's free!

Some PingTool features:

• Ping GUI software with Ping and TCP port check (TCP ping)

• Wake-On-Lan tool (Magic Packet)

• Perform network speedtests in the LAN

• Open/Save configurations

• Create custom actions

• Filter objects

• Automatic logging

• Dashboard for difficult environments

• Repository for large environments

 

Have a look at the short PingTool overview video on YouTube:

Often used TCP ports:

TCP Port 21 - FTP (File Transfer Protocol)
TCP Port 22 - SSH (Secure Shell)
TCP Port 23 - Telnet
TCP Port 25 - SMTP --> article TCP Port Check SMTP 25
TCP Port 80 - HTTP (Hypertext Transfer Protocol)
TCP Port 110 - POP3 (Postoffice Protocol)
TCP Port 135 - DCE / WMI
TCP Port 139 - NetBIOS
TCP Port 143 - IMAP4
TCP Port 443 - HTTPS
TCP Port 445 - SMB
TCP Port 1433 - MS SQL
TCP Port 1494 - ICA (Citrix)
TCP Port 1521 - Oracle
TCP Port 3389 - RDP
TCP Port 5060 - SIP
TCP Port 8080 - HTTP Alternative
TCP Port 8443 - HTTPS Alternative