A Vista PC which was sending SMTP mail fine on Port 25, suddenly stopped. I have fixed the problem by sending on Port 587 - but what I want to know is why? I know this much:
1. It's not the AV/Firewall (PCCillin2007) as turning the software off makes no difference.
2. It's not the WIndows Firewall -it is not turned on.
3. It's not the firewall on the router -XP PCs on the same network can send on Port 25 with no problem. This also shows that it is not the ISP's mail server.
4. I have tried Telneting into the SMTP server. It hangs until you hit Control C - then you get in to the server and communicate with it.
Can anyone tell me what is going on?