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.
Unfortunately, there is nothing we can do to change this. The port #25 is the standard for sending messages and the one eMail Verifier uses. The only solution is switching to another ISP. Smaller local ISP's usually do not block Port 25. Note that if your ISP is Comcast you can update your account to a Business Class account with full access to port 25.
You can check whether you are affected by port 25 blocking right away by using the port25.icannotconnect.com page or the Telnet tool on your computer. It is easier to use the web page above since it gives you the result in a very easy and understandbale form. In case that page doesn't work or you want to double-check you will find below how to do the same test with the Telnet tool on your computer.
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
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: