[opinionator]

I understand how an IP address can belong to reuters.com, but how can it pinpoint the actual sender of the e-mail?

[Talking_Mouse]

They can find out what workstation has that IP address, and at what time the email was sent. Then who was logged onto that workstation at that time was, most probably, who sent the email.

[LibWhacker]

I've been wondering the same thing. This gives some idea of how they do it (subnets and seed data):

Why does your data differ from the WHOIS data?

In any event, this is a nice webpage to know about (look at the map at the bottom): Geobyte's IP Locator