eMail Verifier needs full access to what is called "Port 25", a network connection channel. Some Internet Service Providers (ISP) intercept port 25, so it is not possible to use eMail Verifier nor any software using port 25. Some independent SMTP servers support an additional port other than 25 (26, 587, 2525...) to allow users with authenticated access to connect to them even if port 25 is blocked. Unfortunately eMail Verifier can't use such alternative port as it needs to connect to each address SMTP relay server, not only your server. Relay servers (MTA) only work on port 25. In fact, all e-mail sent via the Internet is routed through port 25. When you run eMail Verifier on your computer, it always uses port 25 to connect to remote e-mail relay servers. Therefore, if your ISP is blocking the port, eMail Verifier will be unable to test e-mail addresses.
A few ISPs are blocking port 25. They are doing this to cut down on the amount of spam that is sent from their networks. All e-mail sent via the Internet is routed through the port 25, the channel used for communication between an e-mail client and an e-mail server. Port 25 blocking allows ISPs to block spam sent out through their networks, but it tends to punish the innocent that have a need to use tools like eMail Verifier or to send through e-mail servers other than those belonging to their ISP. The ISPs that block port 25 require their SMTP server to be used instead of the remote SMTP server or a SMTP server running on your computer.
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. If your ISP is Verizon it appears you have to subscribe to a static IP address service by calling Verizon customer service at (800) 922-0204 (Please check with Verizon if that will indeed remove port 25 block).
You can easily check whether you are affected by port blocking using Telnet. It is explained here: How to find out whether port 25 is blocked by my ISP?