Why do emails from different people have the same IP address upon trace?

[velma]

I am trying to figure out if several emails from different email addresses actualy came from the same person. I put the headers into the tracer and lo and behold the emails do show the same IP. But then, as a cross-check, I traced the header of an email that I know did *not* come from the person in question. And that trace also shows the same IP address. Why would that be?

[Chrispcritters]

Without knowing the IP address I can only guess that they were sent via the same email provider which does not include the sender's IP address so you're seeing the email provider's IP address.