How to find out whether port 25 is blocked by your ISP?

This article explains how to find out whether your ISP is blocking port 25 on your account. Port 25 blocking prevents any 3rd party outgoing mail servers from being accessed while connected to your ISP network via Dialup, DSL or Cable. Many ISP's (Internet Service Providers) are blocking what is called "Port 25" which is the port used to send e-mail. They have chosen to do this to cut down on the amount of "SPAM" mail that is sent. All e-mail sent via the Internet is routed through the port 25, the channel used for communication between a mail client and a mail server.

Hopefully most of SMTP servers offer their service through at least one alternative port in addition to standard port 25. So, if MaxBulk Mailer can't connect to your server through default port 25 try to use port 587 or 2525. If one of those ports work for you you can simply ignore this article.

You can check whether you are affected by port blocking using Telnet.

Telnet is a user command using TCP/IP protocols to access a computer remotely. When you are connected to the network using telnet, you can enter commands and they will be executed as if the were being entered directly onto the server console.

Both Mac OS X and Windows XP operating systems come with Telnet. However, Telnet is not installed with Windows Vista by default, but you can install it by following the following steps: Click the Start button, click Control Panel, click Programs and Features, and then click Turn Windows features on or off. If you are prompted for an administrator password or confirmation, type the password or provide confirmation. In the Windows Features dialog box, select the Telnet Client check box. Click OK. The installation might take several minutes.

On Windows you just need to use the 'Command Prompt' located at Start | Programs | Accesories | Command Promt' and the 'Telnet' command this way:

telnet server_url port

For example:

If port 25 is not blocked you will get a successful 220 response (text may vary):

If port 25 is blocked you will get a connection error or no response at all:

On Mac OS X, you have to use the Terminal tool located in the Applications/Utilities folder and the 'Telnet' command line tool this way:

telnet server_url port

For example:

If port 25 is not blocked you will get a successful 220 response (text may vary):

If port 25 is blocked you will get a connection error:

Written by Stan Busk
for Maxprog, LLC