How to check it port 25 for outgoing email is blocked.

One of my clients is having troubles connecting to their email server when they move from work to home. This is often assumed to be the ISP is blocking outgoing email from using port 25 but I decided to work out how to check if the port 25 for outgoing email is actually blocked.

Often I'll write up instructions but I found the following article was sufficient for my needs.

How to Check if Port 25 is Blocked - HostDime Knowledge Base

The following are the basic steps in case the article is no longer available in future.

Open a CMD window (type cmd and press Enter in the search area)
Type telnet and press Enter (if telnet is installed you'll start Telnet. Type q to exit)

If Telnet is not installed type the following in the cmd window.

Type pkgmgr /iu:”TelnetClient” and press Enter

Now to check if your port 25 is not blocked type the following using your SMTP details for your mail server. The SMTP details have the format smtp.yourdomain or mail.yourdomain.

Type telnet smtp.yourdomain and press Enter (where your domain includes the .com.au or .com)

You will either get a connection which starts with 220, or you'll get a connection error message of some sought.

If connected to quit communication with the mail server type QUIT and press Enter. Then to exit Telnet type q and press Enter.

Hopefully this information will help others do some basic testing.

Kelvin Eldridge
www.OnlineConnections.com.au

Update: 17 August 2021
I'm not a fan of leaving features turned on in Windows 10 so I decided to work out how to turn the Telnet client off. This is done as follows.
Click Windows Start
Click Settings
Select Apps
Select Programs and Features (top right of screen)
Select Turn Windows features on or off (to l of screen eft)
Locate Telnet Client and remove tick
Click OK

Having found this option this can also be used to turn the Telnet client on.