Outlook Express Troubleshooting

I set-up mail accounts on a small, 8-machine network. Initially it worked great. For some reason we can now only receive our email and not send. I get the following message:

Protocol: SMTP, Port: 25, Secure(SSL): No, Socket Error: 10060, ErrorNumber: 0x800CCC0E

What could have changed on the network that I’m overlooking?