Error Msg : Could not open connection to the host, on port 25: connect failed
What does this mean ?
You web application is not able to reach the SMTP Host using Port 25.
1. If Web Server itself relays the email check for SMTP service under Services and as well as under IIS.
2. If Email Relay server is different make sure your web server is able to reach it (by pinging).
2a. Verify with admin, whether Port 25 is open.
3. Check Firewall (software or firewall server) on your server (to allow outgoing connection)
4. Check firewall (software or firewall server) on Relay server (to accept incoming connection from your server).
5. Check the Virus Scanner on your server. Some scanners like MCAFee / Norton disables mass mailing by default.
If you have come across any other scenario please add it in comments.