Most likely, you are being impeded by one or more firewalls.

The first suspect would be the Windows firewall, which exists on WinXP-SP2 onwards. Next, there may be NAT issues, and company firewall/IDS impediments. The ISP may also limit you to sending outbound SMTP from only specified IP addresses, or having specific source and destination domain names in the message.

With this many variables, tackling this is not for the feint of heart. You need decent sniffing tools, and administrative cooperation.

A starting point would be to look at the Windows event log (run eventvwr from a command line, for *nix geeks).